Measurement method and measurement device, and corrosion resistance test method and corrosion resistance test apparatus for coated metal material

ABSTRACT

Provided is a measurement method for measuring a size of expansion of a surface treatment film occurred in a coated metal material that includes a metal base and the surface treatment film provided on the metal base. The measurement method includes the steps of disposing a water-containing material to be in contact with the expansion and an electrode to be in contact with the water-containing material, and electrically connecting between the electrode and the metal base with an external circuit; applying, with the external circuit, a constant voltage between the electrode and the metal base, as a cathode and an anode, respectively, and measuring a current value flowing therebetween; and calculating a size of the expansion, based on the current value measured and a correlation between the current value and the size of the expansion, the correlation being determined on an exploratory basis in advance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2020-107342 filed on Jun. 22, 2020, the entire disclosure of which isincorporated by reference herein.

BACKGROUND

The present disclosure relates to a measurement method and measurementdevice for measuring a size of expansion of the surface treatment film,and a corrosion resistance test method and corrosion resistance testapparatus for the coated metal material.

As a technique for evaluating the performance of coating films, theaccelerated corrosion test such as a combined cycle test and a saltspray test has been performed.

The accelerated corrosion test requires several months for evaluation.It is thus difficult to simply evaluate, for example, the film qualityof the coating film to be coated on steel sheets made of differentcomponents under different baking conditions and to rapidly optimizecoating conditions. Thus, in the material development, the processcontrol in coating factories, and the quality control relating to therust prevention for vehicles, it is desired to establish a quantitativeevaluation method for rapidly and simply evaluating the corrosionresistance of coated steel sheets.

To achieve this desire, Japanese Unexamined Patent Publication No.S59-48649 describes a corrosion resistance evaluation method for acoating film, in which a DC voltage or direct current is applied to acoated metal continuously or intermittently at a counter electrodethrough a solution, and suitability of the coated metal is determinedfrom the width of a portion of the coating film peeled from a coatingfilm defective portion of the coated metal due to anodic polarization ofthe coated metal.

Further, the inventors of the present application had focused on thefact that the corrosion of the coated metal material progresses due to adamaged portion in the surface treatment film on the metal base, and hasbeen already filed a patent application for an electrochemical corrosionresistance test method simulating such corrosion (e.g., JapaneseUnexamined Patent Publication No. 2019-032171 and Japanese PatentApplication No. 2019-534500).

SUMMARY

The method mentioned above is for testing corrosion resistance of acoated metal material by evaluating a size of expansion of a surfacetreatment film occurred in the coated metal material. However, measuringthe size of the expansion by visual observation causes an error. Thismay reduce reliability of the corrosion resistance test.

Hence, the present disclosure is intended to provide a highly reliable,simple measurement method and measurement device for measuring a size ofexpansion of a surface treatment film with high versatility, and acorrosion resistance test method and corrosion resistance test apparatusfor a coated metal material.

Solution to the Problems

To achieve the foregoing object, the measurement method disclosed hereinis directed to a measurement method for measuring a size of expansion ofa surface treatment film occurred in a coated metal material thatincludes a metal base and the surface treatment film provided on themetal base. The measurement method includes the steps of disposing awater-containing material to be in contact with the expansion and anelectrode to be in contact with the water-containing material, andelectrically connecting between the electrode and the metal base with anexternal circuit; applying, with the external circuit, a constantvoltage between the electrode and the metal base, as a cathode and ananode, respectively, and measuring a current value flowing therebetween;and calculating a size of the expansion, based on the current valuemeasured and a correlation between the current value and the size of theexpansion, the correlation being determined on an exploratory basis inadvance.

For example, in the corrosion resistance test for a coated metalmaterial, a sample with a damaged portion is exposed to a corrosionenvironment, and the progress degree of the corrosion of the coatedmetal material may be evaluated, based on the size of the damagedportion before the test and the size of the expansion of a surfacetreatment film occurred around the damaged portion. In this case, thesize of the expansion of the surface treatment film after the test isrequired to be measured accurately. However, measuring the size of theexpansion by visual observation using an image of the expansion occurredor the like for each test may increase the error in the test.

In the present configuration, a constant voltage is applied between theelectrode and the metal base as a cathode and an anode, respectively.This brings the anode reaction (oxidation reaction) in which the metalbase is dissolved to progress in an expanded portion. Further, a cathodereaction (reduction reaction) in which residual oxygen and the like inthe water-containing material are reduced to generate OH⁻ and the likeprogresses at the electrode. The inventors of the present applicationfound that a current value flowing between the electrode and the metalbase increases linearly with respect to the size of the expansion.Accordingly, the present configuration allows the size of the expansionof the surface treatment film to be measured using the electrochemicaltechnique; this enables the process of the test to be simplified and ameasuring error to be reduced.

The “size of the expansion of the surface treatment film” herein refersto an expansion diameter or expansion area, or a peeling diameter orpeeling area. The “expansion diameter” and the “expansion area” refer tothe diameter and area of the expanded portion of the surface treatmentfilm, respectively. The “peeling diameter” and the “peeling area” referto the diameter and area of a peeled portion which is the exposingsurface of the metal base exposed by peeling the expanded portion of thesurface treatment film after the corrosion resistance test,respectively.

The constant voltage may be less than a theoretical voltage at whichelectrolysis of water occurs to generate hydrogen.

In response to the application of a constant voltage which is equal toor higher than the theoretical voltage at which electrolysis of wateroccurs to generate hydrogen, electrolysis of water progresses along withthe cathode reaction at the electrode. With the progress of theelectrolysis of water, an energy loss occurs due to the generation ofhydrogen. Further, the current value may be unstable due to attachmentof bubbles of hydrogen to the electrode arising from the size and shapeof the electrode, for example. In the present configuration, theapplication of the constant voltage less than the theoretical voltage atwhich electrolysis of water occurs to generate hydrogen enables areduction in the generation of hydrogen, thereby improving accuracy ofmeasurement of the size of the expansion.

The constant voltage may be less than 1.23 V.

The theoretical voltage at which electrolysis of water occurs togenerate hydrogen is known to be 1.23 V (25° C.). In the presentconfiguration, a constant voltage of less than 1.23 V is applied. Thisenables a reduction in the generation of hydrogen due to electrolysis ofwater. Accordingly, the accuracy of the measurement of the size of theexpansion can be improved.

The size of the expansion may be an area of the expansion, and the areaof the expansion may be 0.1 mm² or more to 200 mm² or less.

This configuration allows the area of the expansion to be calculatedaccurately and easily as the size of the expansion.

The surface treatment film may be a resin coating film.

The coated metal material including a metal base and the resin coatingfilm provided as a surface treatment film on the metal base facilitatesprogress of the expansion of the resin coating film between the metalbase and the resin coating film, thereby improving the reliability ofthe corrosion resistance test.

The measurement device disclosed herein is directed to a measurementdevice for measuring a size of expansion of a surface treatment filmoccurred in a coated metal material that includes a metal base and thesurface treatment film provided on the metal base. The measurementdevice includes: an electrode to be in contact with a water-containingmaterial disposed to be in contact with the expansion; an externalcircuit configured to electrically connect between the electrode and themetal base; a current supplier provided on the external circuit andconfigured to supply a constant voltage between the electrode and themetal base as a cathode and an anode, respectively; a current detectorconfigured to detect a current value flowing between the electrode andthe metal base; and a calculator configured to calculate a size of theexpansion, based on the current value detected and a correlation betweenthe current value and the size of the expansion, the correlation beingdetermined on an exploratory basis in advance.

This configuration allows the size of the expansion of the surfacetreatment film to be measured using a device based on theelectrochemical technique, thereby reducing an error.

The constant voltage may be less than a theoretical voltage at whichelectrolysis of water occurs to generate hydrogen.

In the present configuration, a constant voltage that is less than thetheoretical voltage at which electrolysis of water occurs to generatehydrogen is applied. This enables improvement in the accuracy of themeasurement of the size of the expansion.

The constant voltage may be less than 1.23 V.

In the present configuration, a constant voltage of less than 1.23 V isapplied. This enables a reduction in the generation of hydrogen due toelectrolysis of water. Accordingly, the accuracy of the measurement ofthe size of the expansion can be improved.

A corrosion resistance test method for a coated metal material disclosedherein is directed to a corrosion resistance test method for a coatedmetal material that includes a metal base and a surface treatment filmprovided on the metal base. The corrosion resistance test methodincludes the steps of: preparing a coated metal material having one ormore damaged portions reaching the metal base through the surfacetreatment film; disposing a water-containing material to be in contactwith one or two out of the one or more damaged portions and one or twoelectrodes to be in contact with the water-containing material, andelectrically connecting between the electrode and the metal base, orbetween the two electrodes, with an external circuit; measuring a sizeof one or two out of the one or more damaged portions; supplying, withthe external circuit, a current between the electrode and the metalbase, or between one of the two electrodes and the other, as an anodeand a cathode, respectively to expand the surface treatment film aroundthe one or two out of the one or more damaged portions; measuring a sizeof expansion of the surface treatment film using the measurement methodof claim 1; and calculating a progress degree of corrosion of the coatedmetal material, based on the size of the one or two out of the one ormore damaged portions and the size of the expansion.

Metal corrosion is known to progress through an anode reaction(oxidation reaction) of generating free electrons by melting (ionizing)metal that is in contact with water and a cathode reaction (reductionreaction) of reducing dissolved oxygen and the like in water by the freeelectrons to generate a hydroxyl group OH⁻ and the like, occurred inparallel.

In this configuration, a current is supplied between the electrode andthe metal base, or between one of the two electrodes and the other, asan anode and a cathode, respectively.

For the metal base material serving as a cathode, the cathode reactionprogresses in the exposing portion of the damaged portion. For one ofthe electrodes serving as an anode, the cathode reaction progresses inthe exposing portion of the metal base at the damaged portion positionednear the electrode serving as an anode via the water-containingmaterial. In either case, electrolysis may also progress depending onthe current supply conditions to generate hydrogen.

With the progress of the cathode reaction, OH⁻ is generated. This bringsthe area around the damaged portion to be in an alkaline environment.This damages the under-treated surface (chemically converted surface) ofthe metal base, thereby reducing adherence of the surface treatment film(simply reducing adherence between the metal base and the surfacetreatment film for no treatment performed on the surface of the metalbase). Accordingly, the surface treatment film is expanded around thedamaged portion. Further, a hydrogen gas generated by electrolysis ofwater and reduction of H⁺ accelerates the expansion of the surfacetreatment film. Such progress of the cathode reaction and expansion ofthe surface treatment film are accelerated reproduction of actualcorrosion of the coated metal material. Accordingly, by comparing thesize of the damaged portion before the current supply and the size ofthe expansion of the surface treatment film occurred around the damagedportion to check the progress degree of the expansion, the corrosionprogression rate of the coated metal material can be determined.

In this configuration, the size of the expansion of the surfacetreatment film is measured by the measurement method mentioned above insuch as a corrosion resistance test. This enables a reduction inmeasuring error of the size of the expansion. Accordingly, thereliability of the corrosion resistance test can be improved.

The corrosion resistance test method may further include the step ofcorrecting the calculated progress degree of the corrosion of the coatedmetal material, based on the size of the one or two out of the one ormore damaged portions and a correlation between the size of the damagedportion and the progress degree of the corrosion of the coated metalmaterial, the correlation being determined on an exploratory basis inadvance.

A variation in the size of the damaged portion before the current supplycauses variations in the progress degree of the cathode reaction andelectrolysis of water which progress at the damaged portion, the degreeof closure of the damaged portion due to expansion of the surfacetreatment film, the degree of degassing of hydrogen generated in theexpanded surface treatment film, and the like. This may further cause avariation in the size of the expansion of the surface treatment film,resulting in a reduction in the reliability of the corrosion resistancetest.

Thus, in the present configuration, the progress degree of the corrosionof the coated metal material calculated is corrected, based on the sizeof the damaged portion measured, and a correlation between the size ofthe damaged portion and the progress degree of the corrosion of thecoated metal material. The correlation is determined on an exploratorybasis in advance. This allows accurate evaluation of the progress degreeof the corrosion of the coated metal material regardless of the size ofthe damaged portion before the current supply. Accordingly, thereliability and versatility of the corrosion resistance test can beenhanced.

The “size of the damaged portion” herein refers to the size of thedamaged portion in a plan view, and is, for example, the area ordiameter of the damaged portion. For a circular damaged portion in aplan view, the area of the damaged portion is given by the area of thecircle. The diameter of the damaged portion is given by the maximumwidth of the damaged portion. The size of the damaged portion herein isassumed to be the same as the size of the exposing portion of the metalbase at the damaged portion.

The progress degree of corrosion may be a rate of increase in the sizeof the expansion of the surface treatment film.

The rate of increase in the size of the expansion of the surfacetreatment film corresponds to the corrosion progress rate. Accordingly,the rate of increase in the size of expansion of the surface treatmentfilm obtained as the progress degree of corrosion of the coated metalmaterial enables highly reliable corrosion resistance test to beperformed.

The one or more damaged portions may be one or more artificially damagedportions.

For example, a damage portion may be made artificially in the corrosionresistance test to bring corrosion to accelerate. The presentconfiguration allows effective measurement of the size of the expansionof the surface treatment film occurred at such an artificially damagedportion. Accordingly, the reliability of the corrosion resistance testcan be improved.

The one or more artificially damaged portions may be in a dot shape in aplan view.

In this configuration, the artificially damaged portion is in a dotshape in a plan view. This allows the surface treatment film to beexpanded effectively in a dome shape in response to the corrosion when asample with the artificially damaged portion is exposed to the corrosionenvironment in the corrosion resistance test mentioned above, forexample. Accordingly, the corrosion in such a corrosion resistance testcan be accelerated.

A corrosion resistance test apparatus for a coated metal materialdisclosed herein is directed to a corrosion resistance test apparatusfor a coated metal material that includes a metal base and a surfacetreatment film provided on the metal base. The corrosion resistance testapparatus includes: one or two electrodes to be in contact with awater-containing material disposed to be in contact with one or two outof one or more damaged portions reaching the metal base through thesurface treatment film; an external circuit configured to electricallyconnect between the electrode and the metal base, or between the twoelectrodes; an additional measurement device for measuring a size of theone or two out of the one or more damaged portions; a current supplierprovided on the external circuit and configured to supply a currentbetween the electrode and the metal base, or between one of the twoelectrodes and the other, as an anode and a cathode, respectively toexpand the surface treatment film around the one or two out of the oneor more damaged portions; the measurement device with any one of theconfigurations mentioned above, for measuring the size of expansion ofthe surface treatment film; and a calculator configured to calculate aprogress degree of corrosion of the coated metal material, based on thesize of the one or two out of the one or more damaged portions and thesize of the expansion.

In this configuration, the size of the expansion of the surfacetreatment film is measured by the measurement device mentioned above.This enables a reduction in measuring error of the size of the damagedportion.

The corrosion resistance test apparatus may further include a correctorconfigured to correct the progress degree of the corrosion of the coatedmetal material calculated by the calculator, based on the size of theone or two out of the one or more damaged portions and a correlationbetween the size of the damaged portion and the progress degree of thecorrosion of the coated metal material, the correlation being determinedon an exploratory basis in advance.

In this configuration, the calculated progress degree of the corrosionof the coated metal material is corrected using the corrector. Thisallows accurate evaluation of the progress degree of the corrosion ofthe coated metal material regardless of the size of the damaged portionbefore the current supply. Accordingly, the reliability and versatilityof the corrosion resistance test can be enhanced.

Advantages of the Invention

As mentioned above, the present disclosure allows the size of theexpansion of the surface treatment film to be measured using theelectrochemical technique; this enables the process of the test to besimplified and a measuring error to be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example corrosion resistance test apparatusaccording to a first embodiment.

FIG. 2 is a cross-sectional view taken along line A-A shown in FIG. 1 .

FIG. 3 illustrates the principle of a corrosion resistance testaccording to the first embodiment.

FIG. 4 is a flowchart of a corrosion resistance test method according tothe first embodiment.

FIG. 5 is a table showing experimental results on a cleaning step.

FIG. 6 is a graph showing experimental results on a first measurementstep.

FIG. 7 is a graph showing other experimental results on the firstmeasurement step.

FIG. 8 is a graph showing yet other experimental results on the firstmeasurement step.

FIG. 9 illustrates how an electrodeposition coating film expands arounda damaged portion.

FIG. 10 is a table showing results of the corrosion resistance test inExperimental Examples and Reference Examples.

FIG. 11 is a graph showing experimental results on a second measurementstep.

FIG. 12 is a graph showing other experimental results on the secondmeasurement step.

FIG. 13 is a graph showing yet other experimental results on the secondmeasurement step.

FIG. 14 illustrates the principle of a corrosion resistance testaccording to a second embodiment.

FIG. 15 is a graph showing a relationship between the diameter of adamaged portion and an index of the corrosion progress rate in acorrosion resistance test according to a third embodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure will now be described withreference to the drawings. The following description of the embodimentsis merely an example in nature, and is not intended to limit the scope,applications, or use of the present disclosure.

First Embodiment

FIGS. 1 and 2 illustrate an example corrosion resistance test apparatusaccording to the present embodiment, which is for a coated metalmaterial. FIG. 3 illustrates the principle of a corrosion resistancetest method according to the present embodiment. In FIGS. 1 to 3 , thereference numeral 1 represents the coated metal material, the referencenumeral 300 represents an electrode portion device, and the referencenumeral 100 represents the corrosion resistance test apparatus.

<Coated Metal Material>

Examples of the coated metal material targeted for the corrosionresistance test according to the present embodiment include a coatedmetal material including a metal base and a resin coating film providedas a surface treatment film on the metal base.

The metal base may be, for example, a steel material for forming anelectric household appliance, a building material, or an automobilepart, such as a cold-rolled steel plate (SPC), a galvanized alloy steelsheet (GA), a high-tensile strength steel sheet, or a hot stampingmaterial, or may be a light alloy material. The metal base may include,on its surface, a chemical conversion coating (e.g., a phosphatecoating, such as a zinc phosphate coating, or a chromate coating).

Specific examples of the resin coating film include cationicelectrodeposition coating films (undercoat films) based on an epoxyresin, an acrylic resin, and the like.

The coated metal material may include a multilayer film of two or morelayers as the surface treatment film. Specifically, for example, for thesurface treatment film being a resin coating film, the coated metalmaterial may be a multilayered coating film obtained by overlaying atopcoat film on an electrodeposition coating film or by overlaying anintermediate coating film and a topcoat film on an electrodepositioncoating film.

The intermediate coating film serves to secure reliable finishing andchipping resistance of the coated metal material and to improveadherence between the electrodeposition coating film and the topcoatfilm. The topcoat film secures reliable color, finishing, and weatherresistance of the coated metal material. Specifically, these coatingfilms may be made from, for example, a paint containing: a base resin,such as a polyester resin, an acrylic resin, and an alkyd resin; and acrosslinking agent, such as a melamine resin, a urea resin, and apolyisocyanate compound (including a blocked polyisocyanate compound).

This configuration allows, for example, taking out of parts from themanufacturing line in each coating step and check of the qualities ofthe coating films, in a manufacturing process of an automobile member.

A coated metal material 1 including: a metal base that includes a steelsheet 2 and a chemical conversion coating 3 on the steel sheet 2; and anelectrodeposition coating film 4 (resin coating film) provided as asurface treatment film on the metal base will be described below as anexample.

As illustrated in FIGS. 2 and 3 , the coated metal material 1 has onedamaged portion 5 reaching the steel sheet 2 through theelectrodeposition coating film 4 and the chemical conversion coating 3.The damaged portion 5 may be artificially made or naturally made. Aplurality of damaged portions 5 may be made apart from each other. Inthis case, one damaged portion 5 means one of the damaged portions 5.

<Water-Containing Material>

A water-containing material 6 contains water and a supportingelectrolyte, and functions as a conductive material. Thewater-containing material 6 may be a muddy material further containing aclay mineral. For the water-containing material 6 further containing aclay mineral, ions and water in the water-containing material 6 easilypermeate through a portion of the electrodeposition coating film 4around the damaged portion 5 in the holding step S7 and current supplystep S9, which will be described later.

The supporting electrolyte is a salt and is for imparting sufficientelectrical conductivity to the water-containing material 6. Thesupporting electrolyte may be at least one salt selected from sodiumchloride, sodium sulfate, calcium chloride, calcium phosphate, potassiumchloride, potassium nitrate, potassium hydrogen tartrate, and magnesiumsulfate. The supporting electrolyte may be particularly preferably atleast one salt selected from sodium chloride, sodium sulfate, andcalcium chloride. The water-containing material 6 contains thesupporting electrolyte preferably at 1 mass % or more to 20 mass % orless, more preferably at 3 mass % or more to 15 mass % or less,particularly preferably at 5 mass % or more to 10 mass % or less.

The clay mineral is for making the water-containing material 6 intomuddy material and promoting the movement of ions and permeation ofwater into the electrodeposition coating film 4 to accelerate progressof corrosion in the current supply step S9. The clay mineral may be, forexample, a layered silicate mineral or zeolite. The layered silicatemineral may be, for example, at least one selected from kaolinite,montmorillonite, sericite, illite, glauconite, chlorite, and talc. Outof these, kaolinite may be particularly preferably employed. Thewater-containing material may contain a clay mineral preferably at 1mass % or more to 70 mass % or less, more preferably at 10 mass % ormore to 50 mass % or less, particularly preferably at 20 mass % or moreto 30 mass % or less. The water-containing material 6 being muddymaterial is allowed to be provided even on a non-horizontal surface ofthe electrodeposition coating film 4.

The water-containing material 6 may contain an additive in addition towater, the supporting electrolyte, and the clay mineral. Specificexamples of the additive include organic solvents such as acetone,ethanol, toluene, and methanol, and substances for improving wettabilityof the coating film. These organic solvents, substances, and the likecan also function to promote permeation of water into theelectrodeposition coating film 4. Any of these organic solvents,substances, and the like may be added to the water-containing material 6as a substitute for the clay mineral. For the water-containing material6 containing an organic solvent, the content of the organic solvent ispreferably 5% or more to 60% or less relative to the content of water interms of volume ratio. The volume ratio is preferably 10% or more to 40%or less, more preferably 20% or more to 30% or less.

<Corrosion Resistance Test Apparatus>

The corrosion resistance test apparatus 100 includes an electrodeportion device 300, an external circuit 7 (a treatment device, a firstmeasure device, and a second measurement device), a current supplier 8(a treatment device, a current detector, a first measurement device, asecond measurement device), a control device 9 (a calculator, acorrector, a temperature controller, a treatment device, a firstmeasurement device, a second measurement device), an optional rubberheater 41 (a first temperature control element), and an optional hotplate 43 (a second temperature control element).

<<Electrode Portion Device>>

The electrode portion device 300 is for use in a corrosion resistancetest according to the present embodiment, and includes a container 30,an electrode 12 (a treatment device, a first measurement device, asecond measurement device), and an optional temperature sensor 37 (atemperature detector).

—Container—

The container 30 is placed on the electrodeposition coating film 4 ofthe coated metal material 1. The container 30 includes a container body31, an optional bottom portion 32, a lid 34, a through hole 38, and anoptional hole 36.

[Container Body and Bottom Portion]

The container body 31 and the bottom portion 32 are each a member in atubular shape such as a cylindrical shape and a polygonal tubular shape,and are each preferably a member in a cylindrical shape in order toreduce strain when thermally expanded.

The bottom portion 32 is in contact with the surface of theelectrodeposition coating film 4 via the bottom surface 32A. Thecontainer body 31 is disposed opposite to the bottom surface 32A in thebottom portion 32.

The container body 31 has the same inner diameter as the bottom portion32. The inside of the space defined by the inner circumferentialsurfaces of the container body 31 and the bottom portion 32 constitutesa water-containing material holder 11 for holding a water-containingmaterial. The water-containing material holder 11 has an opening 11Aprovided in the bottom surface 32A. A region of the coated metalmaterial 1 defined by the opening 11A serves as a measurement targetportion 4A with the container 30 placed on the electrodeposition coatingfilm 4 of the coated metal material 1.

The water-containing material 6 is in contact with the surface of theelectrodeposition coating film 4 and enters the damaged portion 5, withthe water-containing material 6 contained in the water-containingmaterial holder 11.

The bottom portion 32 is a sheet-like sealing material made from asilicone resin, for example, and is used to improve adherence betweenthe container body 31 and the electrodeposition coating film 4, and fillthe gap therebetween, when the container 30 is placed on the coatedmetal material 1. This can effectively reduce leaking of thewater-containing material 6 from the gap between the container body 31and the electrodeposition coating film 4. The bottom portion 32 ispreferably provided in order to sufficiently reduce leaking of thewater-containing material 6, although it may not be provided.

In order to effectively reduce leaking of the water-containing material6, the bottom portion 32 has a thickness of preferably more than 1 mm,and a hardness of preferably 50 or less as a type A durometer hardnessdefined in JIS K 6250, as shown in experimental examples to be describedlater. The upper limit of the thickness of the bottom portion 32 may be,but is not particularly limited to, for example, 10 mm or less, in orderto obtain an advantage of the attractive force of a magnet 33, whichwill be described later, and to reduce a cost of the material for thebottom portion 32. The lower limit of the hardness of the bottom portion32 may be, but is not particularly limited to, for example, 10 or moreas a type A durometer hardness, in order for a product usable as thebottom portion 32 to be easily available.

The container body 31 may be made from a resin material, such as anacrylic resin, an epoxy resin, and aromatic polyether ether ketone(PEEK) or from ceramic, particularly preferably made from at least oneresin material selected from the group consisting of an acrylic resin,an epoxy resin, and an aromatic polyether ether ketone (PEEK). Thisallows a reduction in the weight and cost of the electrode portiondevice 300, in turn, the corrosion resistance test apparatus 100, whilesecuring insulation between the container body 31 and the outside.

The container body 31 includes a large-diameter portion 302 in thevicinity of the bottom portion 32, and a small-diameter portion 301disposed opposite to the bottom portion 32 relative to thelarge-diameter portion 302. The large-diameter portion 302 andsmall-diameter portion 301 have the same inner diameter around thecenter axis 31B of the container body 31. The large-diameter portion 302has an outer diameter larger than the small-diameter portion 301. Theouter circumferential surfaces of the large-diameter portion 302 andsmall-diameter portion 301 are in connection with each other via a stepportion 303.

The inner diameters of the container body 31 and bottom portion 32,i.e., the diameter of the water-containing material holder 11, aresuitably larger than the damaged portion 5. The container 30 is suitablyplaced on the electrodeposition coating film 4 such that thewater-containing material holder 11 is concentric with the damagedportion 5. The container 30 having the foregoing configuration cancontain a sufficient amount of the water-containing material 6 requiredfor the corrosion resistance test while the water-containing material 6covers the entire damaged portion 5. For example, for the damagedportion 5 having a diameter of 0.1 mm or more to 7 mm or less, thediameter of the water-containing material holder 11 may be, for example,0.5 mm or more to 45 mm or less, preferably 0.5 mm or more to 30 mm orless. The container 30 having this configuration can contain asufficient amount of the water-containing material 6 required for thecorrosion resistance test while the water-containing material 6 coversthe entire damaged portion 5.

A portion of the large-diameter portion 302 in the vicinity of thebottom portion 32 has a groove 304. The groove 304 is positioned arounda portion of the water-containing material holder 11 near the opening11A, and contains a ring-shaped magnet 33 therein. Thus, the container30 is attracted and fixed to the coated metal material 1 by theattractive force of the magnet 33 while the container 30 is placed onthe electrodeposition coating film 4 of the coated metal material 1.This can effectively reduce the displacement of the container 30, andcan improve the reliability of the corrosion resistance test to bedescribed below.

The magnet 33 may be, for example, a ferrite magnet, a neodymium magnet,a samarium-cobalt magnet, but is suitably a neodymium magnet or asamarium-cobalt magnet, in order to obtain a high attractive force. Theintensity of the magnet 33 is preferably 370 mT or higher, as shown inexperimental examples to be described later. This configuration cansecure higher adherence between the electrode portion device 300 and thecoated metal material 1. The upper limit of the intensity of the magnet33 may be, but is not particularly limited to, for example, 1300 mT orlower.

The magnet 33 is suitably sealed with, for example, an epoxy resin afterbeing placed in the groove 304. This can reduce removal of the magnet 33from the groove 304, and leaking of the water-containing material 6through the water-containing material holder 11 into the groove 304, forexample. In addition, the sealing secures insulation between the magnet33 and the water-containing material 6. This substantially prevents areduction in the reliability of the corrosion resistance test due todissolution of highly conductive components of the magnet 33 into thewater-containing material 6.

Experimental Examples

A silicone mat serving as a bottom portion 32 made from a silicone resinwas placed in a portion of the container body 31 made from an epoxyresin (the inner diameter of the water-containing material holder 11, 10mm) in the vicinity of the bottom surface 32A, which was then placed ona flat table. Subsequently, water was introduced into the container body31, which was then held for 10 minutes. Thereafter, the presence orabsence of water leakage was checked. A ring-shaped neodymium magnet(manufactured by Magfine Corporation) had been embedded, using an epoxyresin, in a portion of the container body 31 in the vicinity of thebottom surface 32A. Table 1 shows the results. The hardness of thesilicone mat was indicated by the type A durometer hardness defined inJIS K 6250.

TABLE 1 Experimental Examples 1 2 3 4 Magnet Intensity (mT) 367 367 380380 Attractive Force (kgf) 4.4 4.4 5.0 5.0 Silicone Mat Hardness (Type A70 50 50 50 (Bottom Durometer Hardness) Portion) Thickness (mm) 0.5 0.51 1.5 Presence or Absence of Water Leakage Present Present PresentAbsent

The results of Experimental Examples 1 to 4 demonstrate that waterleakage can be more effectively reduced when the intensity of the magnetis higher, the hardness of the silicone mat is lower, and the thicknessof the silicone mat is higher.

[Lid]

The lid 34 closes the upper opening 31A of the container body 31. Thevolatilization of a solvent component of the water-containing material 6during the corrosion resistance test changes the concentration of acomponent of the water-containing material 6, which may reduce thereliability of the test. The lid 34 closing the upper opening 31Areduces releasing of a volatile component of the water-containingmaterial 6 moving upward in the container body 31 to the outside.Accordingly, the reduction in water-containing material 6 during thetest can be substantially prevented. Further, for the test performedwith an increase in temperatures of the water-containing material 6 andthe coated metal material 1, the temperature keeping efficiency can beincreased.

Similarly to the container body 31, the lid 34 may be made from a resinmaterial, such as an acrylic resin, an epoxy resin, and aromaticpolyether ether ketone (PEEK) or from ceramic, particularly preferablymade from at least one resin material selected from the group consistingof an acrylic resin, an epoxy resin, and an aromatic polyether etherketone (PEEK). This allows reduction in the weight and cost of theelectrode portion device 300, in turn, the corrosion resistance testapparatus 100, while securing insulation between the water-containingmaterial holder 11 and the outside.

In particular, the use of the PEEK material as a material for thecontainer body 31 and/or the lid 34 allows a reduction in erosion of thecontainer body 31 and/or the lid 34 due to a malfunction of the rubberheater 41 and/or the hot plate 43 or other issues.

The container body 31 and the lid 34 may be made from differentmaterials or the same material. The container body 31 and the lid 34 maybe integral with or separate from each other.

[Through Hole]

The through hole 38 is a hole for releasing the internal pressure of thecontainer 30, provided in the upper side wall of the container body 31,that is, a side wall above the upper surface of the water-containingmaterial 6 in the container body 31 so as to penetrate the side wall.During the corrosion resistance test, gases such as hydrogen may begenerated through chemical reaction. In such case, complete sealing ofthe container body 31 increases the internal pressure of the container30, which may lead to breakage of the container 30 and other issues. Inthe present configuration, gases generated during the test are removedthrough the through hole 38. This substantially prevents the increase inthe internal pressure of the container 30. Moreover, the through hole 38is provided in the upper side wall of the container body 31. Thisreduces leaking of the water-containing material 6, releasing of thevolatile component of the water-containing material 6, and other issues,compared with the case where the through hole 38 is provided in thelower side wall (a portion of the side wall below the upper surface ofthe water-containing material 6 in the container body 31), the lid 34,or the like.

The through hole 38 may also be used for pulling out the electrode 12 orwiring 71 of the external circuit 7 and/or for introducing thewater-containing material 6.

The number of the through holes 38 may be one or more. The number of thethrough holes 38 is preferably one, two, or three. For one through hole38, the through hole 38 is used for the three purposes. This simplifiesthe configuration of the electrode portion device 300 and requires a fewthrough holes 38, thereby allowing the effective reduction in releasingof the volatile component of the water-containing material 6. For two orthree through holes 38, the through holes 38 may share the threepurposes. This facilitates operations for the three purposes.

The shape of the through hole 38 used for releasing the internalpressure is not particularly limited, but the through hole 38 for theother purposes is suitably a straight hole having a circular crosssection and a constant diameter in order to facilitate operation.

The water-containing material 6 may be introduced into thewater-containing material holder 11 with a dropper or a syringe, forexample. Considering this, the through hole 38 which may be used forintroducing the water-containing material 6 is suitably tilted downwardfrom the outside of the container body 31 toward its inside, asillustrated in FIG. 2 . This facilitates introduction of thewater-containing material 6.

The diameter of the through hole 38, i.e., the maximum width in thecross section perpendicular to the center axis of the through hole 38 ispreferably 1 mm or more to 7 mm or less, more preferably 2 mm or more to5 mm or less. For a large amount of gases generated, the through hole 38having a diameter less than the lower limit may cause insufficientrelease of the internal pressure of the container 30, or may causedifficulty in the use for the other purposes. The through hole 38 havinga diameter more than the upper limit may excessively release a volatilecomponent of the water-containing material 6 therethrough.

[Hole]

Preferably, a hole 36 for allowing a temperature sensor 37 to beinserted therein is provided in the lower side wall of the containerbody 31, that is, a portion of the side wall below the upper surface ofthe water-containing material 6 in the container body 31.

The bottom 36A of the hole 36 is penetrating the container body 31 tothe inside. This enables the distal end 37A of the temperature sensor 37inserted into the hole 36 to enter the inside of the water-containingmaterial holder 11 though the bottom 36A to be in contact with thewater-containing material 6. Accordingly, the temperature sensor 37 candetect the temperature of the water-containing material 6.

The hole 36 is suitably such that its bottom 36A becomes close to theelectrodeposition coating film 4 as much as possible when the container30 is disposed on the electrodeposition coating film 4.

Specifically, for example, such a hole 36 may be provided in the sidewall of the container body 31 using a mold when the container body 31 isformed. In addition, the hole 36 may be provided by embedding a tubularmember made from an insulating material such as a resin with a highthermal conductivity and a ceramic, in the side wall of the containerbody 31 using insert molding when the container body 31 is formed.

Note that the hole 36 may be provided so that the bottom 36A does notpenetrate the container body 31 to the inside.

—Temperature Sensor—

The electrode portion device 300 preferably includes a temperaturesensor 37 for detecting the temperature of the water-containing material6. The temperature sensor 37 is inserted into the hole 36 to detect thetemperature of the water-containing material 6.

In the corrosion resistance test according to the present embodiment,the temperature of the water-containing material 6, particularly portionof the water-containing material 6 near the electrodeposition coatingfilm 4 is important. The temperature sensor 37 inserted into the hole 36can accurately detect the temperature of the portion of thewater-containing material 6 near the electrodeposition coating film 4,thereby improving the reliability of the corrosion resistance test.

Specific examples of the temperature sensor 37 include a thermocouple, afiber optic thermometer, and an infrared thermometer. With beinginserted into the hole 36, the temperature sensor 37 is preferablymolded with a resin having a high thermal conductivity or anothermaterial in order to further accurately detect the temperature of thewater-containing material 6.

An amount of the distal end 37A of the temperature sensor 37 enteringthe inside of the container body 31 is suitably as small as possible.This can substantially prevent the reduction in accuracy of thedetection of the temperature due to adhering of the electrodepositioncoating film 4 expanded in the current supply step S9 to be describedlater to the distal end of the temperature sensor 37.

For the electrode portion device 300 having no hole 36 and notemperature sensor 37, a thermometer may be placed in thewater-containing material holder 11 to measure the temperature of thewater-containing material 6.

—Electrode—

The electrode 12 is provided with its distal end 12 a being sunk in thewater-containing material 6 and thus is in contact with thewater-containing material 6.

Specific examples of the electrode 12 include a carbon electrode and aplatinum electrode.

The electrode 12 may be in a shape commonly used in electrochemicalmeasurement, but is preferably a perforated electrode having at leastone hole at its distal end 12 a. The distal end 12 a is preferablydisposed such that the hole is substantially parallel to the surface ofthe electrodeposition coating film 4. For example, a perforatedelectrode has a ring-shaped distal end 12 a, and is provided such thatthe ring faces the electrodeposition coating film 4. Alternatively, amesh electrode may be employed as the perforated electrode. The meshelectrode may be disposed to be substantially parallel with theelectrodeposition coating film 4 with being sunk in the water-containingmaterial 6.

The cleaning step S4 and current supply step S9, which will be describedlater, may cause hydrogen to be generated at the damaged portion 5. Thehydrogen is removed through the hole provided in the distal end 12 a,thereby avoiding retention of the hydrogen between the electrode 12 andthe electrodeposition coating film 4. In this way, it is possible toavoid deterioration of the electrical conductivity.

<<External Circuit>>

The external circuit 7 includes a wiring 71 and a current supplier 8(current detector) disposed on the wiring 71. The wiring 71 electricallyconnects between the electrode 12 and the steel sheet 2. The wiring 71may be of any known type.

—Current Supplier—

The current supplier 8 serves as a power supply that supplies avoltage/current between the electrode 12 and the steel sheet 2particularly in the cleaning step S4, first measurement step S5, currentsupply step S9, and second measurement step S10, which will be describedlater. The current supplier 8 also serves as a current detector/voltagedetector that detects a current/voltage flowing between them. Specificexamples of the current supplier 8 include a potentiostat/galvanostatthat can control an applied voltage/current.

The current supplier 8 is electrically or wirelessly connected to thecontrol device 9 to be described later, and is controlled by the controldevice 9. Current supply information such as a voltage value, a currentvalue, time for current supply, and other parameters applied from thecurrent supplier 8 to the external circuit 7 or detected by the currentsupplier 8 are transmitted to the control device 9.

<<Rubber Heater and Hot Plate>>

The rubber heater 41 is for adjusting the temperature of or warming thewater-containing material 6 in the water-containing material holder 11.The rubber heater 41 covers the outer circumferential surface 301A(outer circumferential portion) of the small-diameter portion 301 of thecontainer body 31 and is disposed in the outer circumferential portionof the container body 31, specifically, disposed on the step portion 303of the large-diameter portion 302. The rubber heater 41 is bonded andfixed to the outer circumferential surface 301A of the small-diameterportion 301 with, for example, an adhesive tape or any other similarmaterial. In FIGS. 1 and 2 , only a portion of the rubber heater 41 isshown in order to clearly show the hole 36 and the temperature sensor37. The first temperature control element may be, for example, a filmheater as a substitute for the rubber heater 41.

The hot plate 43 is disposed on the side of the coated metal material 1opposite to the container 30, that is, on the steel sheet 2 of thecoated metal material 1. The hot plate 43 is for adjusting thetemperatures of or warming the coated metal material 1 and portion ofthe water-containing material 6 near the electrodeposition coating film4 from the back side of the coated metal material 1. The secondtemperature control element may be, for example, a Peltier element as asubstitute for the hot plate 43.

The rubber heater 41 and the hot plate 43 are electrically or wirelesslyconnected to the control device 9 to be described later. The controlunit 93 of the control device 9 serves as a temperature controller tocontrol the temperatures of the rubber heater 41 and the hot plate 43.In this way, the control unit 93 can be configured to adjust thetemperatures of or warm the coated metal material 1 and thewater-containing material 6. As described above, the rubber heater 41and the hot plate 43 are suitably controlled by a single temperaturecontroller. In other words, the temperature controller connected to therubber heater 41 suitably also serves as the temperature controllerconnected to the hot plate 43. This contributes to the downsizing of thecorrosion resistance test apparatus 100. This configuration is notintended to limit the use of devices other than the control device 9 asa temperature controller. The temperatures of the rubber heater 41 andthe hot plate 43 may be controlled by different temperature controllers.

This configuration allows appropriate adjustment of the temperatures ofor appropriate warming of the water-containing material 6 and the coatedmetal material 1. Thus, in the corrosion resistance test to be describedlater, movement of ions to and permeation of water into theelectrodeposition coating film 4 is promoted, and corrosion of thedamaged portion 5 can be effectively progressed. This allows thecorrosion resistance test to be performed in a shorter time with higherreliability. Further, the temperature of the water-containing material 6and the coated metal material 1 can be kept constant over desiredtesting time. This allows the corrosion resistance test to be performedunder a predetermined temperature condition with higher reliability.

Either one of the rubber heater 41 or the hot plate 43 may be provided,but the hot plate 43 is preferably provided in order to accuratelycontrol the temperature of portion of the water-containing material 6near the electrodeposition coating film 4.

Even for both the rubber heater 41 and the hot plate 43 provided, bothof or either one of the temperatures of the water-containing material 6and the coated metal material 1 may be adjusted. Both of thetemperatures of the coated metal material 1 and the water-containingmaterial 6 are suitably adjusted in order to uniformize theirtemperature distributions.

<<Control Device>>

The control device 9 is based on, for example, a known microcomputer,and includes an arithmetic unit 91, a storage 92, and a control unit 93.The control device 9 may further include a display unit such as adisplay, and an input unit such as a keyboard, although not shown. Thestorage 92 stores pieces of information such as various pieces of dataand arithmetic processing programs. The arithmetic unit 91 performsvarious kinds of arithmetic processing based on the information storedin the storage 92, information input with the input unit, and otherinformation. The control unit 93 outputs a control signal to the targetto be controlled to perform various kinds of controls based on the datastored in the storage 92, an arithmetic result of the arithmetic unit91, and the like.

As mentioned above, the control device 9 is electrically or wirelesslyconnected to the current supplier 8, the rubber heater 41, the hot plate43, and the temperature sensor 37.

As mentioned above, the current supply information detected with thecurrent supplier 8, the temperature information detected with thetemperature sensor 37, and other information are transmitted to thecontrol device 9 and is stored in the storage 92. The control unit 93outputs a control signal to the current supplier 8, the rubber heater41, and the hot plate 43 to control a voltage value/current valueapplied from the current supplier 8 to the external circuit 7 and thetemperature settings of the rubber heater 41 and the hot plate 43. Thecontrol unit 93 may be configured to control the temperature settings ofthe rubber heater 41 and the hot plate 43 based on the temperatureinformation detected with the temperature sensor 37. This configurationallows the temperatures to be controlled further accurately.

The arithmetic unit 91 functions as a calculator that calculates thesizes of the damaged portion 5 and the expansion of theelectrodeposition coating film 4 in the first measurement step S5 andsecond measurement step S10, which will be described later. The storage92 stores various pieces of information on the correlation used for thecalculation and information on the sizes of the damaged portion 5 andthe expansion of the electrodeposition coating film 4.

The arithmetic unit 91 functions also as a calculator that calculatesthe progress degree of corrosion of the coated metal material 1 in thecalculation step S11 to be described later. The storage 92 furtherstores information on the calculated progress degree of the corrosion ofthe coated metal material 1.

<<First Measurement Device and Second Measurement Device>>

Although will be described in detail, the electrode 12, the externalcircuit 7, the current supplier 8, and the control device 9 constituteeach of the first measurement device (additional measurement device)that measures the size of the damaged portion in the first measurementstep S5 to be described later, and a second measurement device(measurement device) that measures the size of the expansion of theelectrodeposition coating film 4 in the second measurement step S10 tobe described later.

In this embodiment, the first and second measurement devices each areconstituted by the electrode 12, the external circuit 7, the currentsupplier 8, and the control device 9, and thus have the sameconfiguration but may have different configurations. The first andsecond measurement devices suitably have the same configuration in orderto unify the accuracies of the measured values of both the measurementdevices, improve the accuracy of calculation of the progress degree ofthe corrosion, and contribute to downsizing of the corrosion resistancetest apparatus 100.

<<Damaged Portion Treatment Device>>

Although will be described in detail, the electrode 12, the externalcircuit 7, the current supplier 8, and control device 9 constitute thetreatment device that cleans the damaged portion 5 in the cleaning stepS4 to be described later. The corrosion resistance test apparatus 100according to the present embodiment includes the treatment device,thereby enabling the corrosion resistance test to be performed withoutadherents on the surface of the damaged portion 5. Accordingly, thereliability of the corrosion resistance test can be improved.

<Corrosion Resistance Test Method>

The corrosion resistance test method for the coated metal material 1according to the present embodiment includes, as shown in FIG. 4 , apreparation step S1, a circuit connection step S2 (connection step), afirst placement step S3, an optional cleaning step S4, a firstmeasurement step S5, a second placement step S6 (connection step), anoptional holding step S7, an optional temperature measurement step S8, acurrent supply step S9, a second measurement step S10, a calculationstep S11, and an optional correction step S12. These steps will now bedescribed. The correction step S12 will be described in the thirdembodiment.

<<Preparation Step>>

In the preparation step S1, prepared is a coated metal material 1 havingone damaged portion 5 reaching a steel sheet 2 through anelectrodeposition coating film 4 and a chemical conversion coating 3.

In general, a coated metal material with a coating film starts tocorrode after a corrosion factor such as salt water has permeated intothe coating film and reached the base. The process of the corrosion ofthe coated metal material is divided into a stage until occurrence ofthe corrosion and a stage in which the corrosion progresses. Thecorrosion can be evaluated through determining the period until thecorrosion starts (i.e., a corrosion resistance time) and the rate atwhich the corrosion progresses (corrosion progress rate).

If there is a damaged portion 5 reaching the steel sheet 2 through theelectrodeposition coating film 4 and the chemical conversion coating 3,the water-containing material 6, when comes into contact with thedamaged portion 5, enters the damaged portion 5, and comes into contactwith the exposed portion of the steel sheet 2. The damaged portion 5allows creation of the simulated state at the end of the stage untiloccurrence of the corrosion, that is, at the end of the corrosionresistance time, out of the process of the corrosion of the coated metalmaterial 1. This allows information on the corrosion progress rate to beefficiently obtained in the corrosion resistance test.

As mentioned above, the damaged portion 5 may be naturally damagedportion or an artificially damaged portion, and suitably an artificiallydamaged portion. The artificially damaged portion 5 is allowed to beformed in a desired shape and size to some extent, for example. Thus,for example, in the cleaning step S4 to be described later, a force ofhydrogen pushing up is applied to the entire damaged portion 5, so thatadherents are easily removed. In addition, it becomes easy to measurethe size of the damaged portion 5 in the first measurement step S5.Further, the progress of the expansion of the electrodeposition coatingfilm 4 in the current supply step S9 is facilitated. It also becomeseasy to measure the size of the expansion of the electrodepositioncoating film 4 in the second measurement step S10. Accordingly, thequantitativeness and reliability of the corrosion resistance test can beimproved.

The damaged portion 5 may be a dot-shaped damaged portion, a lineardamaged portion such as a cut made with a cutter, but is preferably adot-shaped damaged portion. The “dot shape” means a shape such as acircular, a polygonal, or the like in a plan view, with a ratio betweenthe maximum width and the minimum width of 2 or less. The dot-shapeddamaged portion 5 allows the electrodeposition coating film 4 to beexpanded effectively in a dome shape in response to the corrosion,thereby allowing the corrosion to be accelerated.

The artificially damaged portion 5 may be formed with any kind of tool.The dot-shaped damaged portion 5 is formed preferably with anartificially damaging punch or an indenter of a Vickers hardness testerat a predetermined load in order not to vary the size and depth of thedot-shaped damaged portions 5, i.e., in order to form the dot-shapeddamaged portion 5 quantitatively. For example, the linear damagedportion 5 other than the dot-shaped damaged portion 5 may be formed witha cutter or another tool.

<<Circuit Connection Step>>

Next, in the circuit connection step S2, a container 30 is disposed onthe electrodeposition coating film 4 of the coated metal material 1placed on the hot plate 43 so as for the water-containing materialholder 11 to surround the damaged portion 5, as illustrated in FIGS. 1and 2 . Then, an electrode 12 connected to one end of the wiring 71 isdisposed in the water-containing material holder 11 through the throughhole 38. The wiring 71 has the other end connected to the steel sheet.Accordingly, the electrode 12 and the steel sheet 2 are beingelectrically connected to each other via an external circuit 7. Atemperature sensor 37 and a rubber heater 41 are further disposed.

The water-containing material holder 11 is preferably provided to beconcentric with the damaged portion 5. In the electrode 12, its distalend 12 a having a hole is provided preferably to be parallel to thesurface of the electrodeposition coating film 4 and to be concentricwith the damaged portion 5.

<<First Placement Step>>

In the first placement step S3, the water-containing material 6particularly used in the subsequent cleaning step S4 is placed at apredetermined amount in the water-containing material holder 11. At thistime, at least the distal end 12 a of the electrode 12 is being sunk inthe water-containing material 6.

The water-containing material 6 contained in the water-containingmaterial holder 11 comes into contact with the surface of theelectrodeposition coating film 4, and enters the inside of the damagedportion 5.

<<Cleaning step>>

Adherents such as the electrodeposition coating film 4 and dirt on thesurface of the damaged portion 5 may reduce acceleration of thecorrosion in the current supply step S9 to be described later even withthe damaged portion 5 reaching the steel sheet 2.

In the cleaning step S4, the current supplier 8 supplies a currentbetween the electrode 12 and the steel sheet 2 while alternatelyswitching the direction of the current flowing through the externalcircuit 7. The switching of the direction of the current using thecurrent supplier 8 is controlled by the control device 9.

The state I in FIG. 3 illustrates the electrode 12 connected to thepositive electrode side of the current supplier 8 and the steel sheet 2connected to the negative electrode side of the current supplier 8. Inthe state I, the oxidation reaction progresses at the interface of theelectrode 12 with the water-containing material 6. Thus, the electrode12 serves as an anode. On the other hand, electrons e⁻ are supplied tothe damaged portion 5 via the steel sheet 2 in the exposing portion 5Aof the steel sheet 2 at the damaged portion 5. Then, a cathode reactionin which dissolved oxygen and the like in water are reduced using theelectrons e⁻ to generate a hydroxyl group OH⁻ progresses at theinterface of the surface of the steel sheet 2 with the water-containingmaterial 6. Thus, the steel sheet 2 serves as a cathode. If applied area voltage which is equal to or higher than the theoretical voltage atwhich electrolysis of water occurs to generate hydrogen or a currentrequiring such voltage, electrolysis of water is also progressed in thesteel sheet 2 to generate hydrogen.

In contract, the state II in FIG. 3 illustrates the electrode 12connected to the negative electrode side of the current supplier 8 andthe steel sheet 2 connected to the positive electrode side of thecurrent supplier 8. In the state II, the reduction reaction progressesat the interface of the electrode 12 with the water-containing material6. Thus, the electrode 12 serves as a cathode. On the other hand, ananode reaction in which the steel sheet 2 is dissolved progresses in theexposing portion 5A of the steel sheet 2 at the damaged portion 5. Thus,the steel sheet 2 serves as an anode.

The alternate switching of the direction of the current applied from thecurrent supplier 8 to the external circuit 7 means alternate switchingbetween the states I and II in FIG. 3 . The alternate switching betweenthe states I and II makes the anode reaction (state II) and the cathodereaction (state I) progress alternately at the damaged portion 5.Specific experimental examples will be described below.

FIG. 5 shows specific experimental results of the cleaning step S4.

First, a steel sheet 2 (SPC) serving as a metal base was provided withan epoxy resin-based electrodeposition coating film 4 (bakingconditions: 150° C.×20 min, thickness: 10 μm) via a chemical conversioncoating 3 (zinc phosphate coating; chemical conversion treatment time,120 sec) to produce a coated metal material 1, which was used asmaterials under test (MUTs) A1 to A4. A damaged portion 5 was formedartificially in the surface of the electrodeposition coating film 4 ineach of the MUTs A1 to A4 using a Vickers hardness tester with a load of30 kg.

A photograph of the MUT A1 shown in FIG. 5 is a digital photomicrographof the untreated MUT A1 after the formation of the damaged portion 5. Ascan be seen from the photograph, the electrodeposition coating film 4 isadhered to the entire surface of the damaged portion 5.

The other MUTs A2 to A4 underwent respective treatments shown in FIG. 5using a 5 mass % salt water (at normal temperature) as awater-containing material 6 with application of a constant current of 5mA after the damaged portion 5 was formed in the same manner as in theMUT A1.

The MUT A2 underwent the treatment (I→II→I→II→I) of repeatedly holdingthe state I (the steel sheet, −; the electrode, +) for 20 sec andholding the state II (the steel sheet, +; the electrode, −) for 20 sec.As can be seen from the photograph of the MUT A2 shown in FIG. 5 , theelectrodeposition coating film 4 on the surface of the damaged portion 5is lifted. In the state I, the cathode reaction progresses in thedamaged portion 5. This generates an alkaline environment near thedamaged portion 5, thereby reducing adherence of the electrodepositioncoating film 4 to the surface of the damaged portion 5, and causeshydrogen generated in the cathode reaction and electrolysis of water topush up the electrodeposition coating film 4. In the state II, the anodereaction progresses at the damaged portion 5. This slightly dissolvesthe surface of the steel sheet 2, and causes further reduction in theadherence of the electrodeposition coating film 4 to the surface of thedamaged portion 5. Further, the alternate repeating of the states I andII reduces adherence of the electrodeposition coating film 4 to thesurface of the damaged portion 5 and causes hydrogen to push up theelectrodeposition coating film 4. This is considered to effectively liftthe electrodeposition coating film 4.

The MUT A3 underwent the treatment of holding the state II (the steelsheet, +; the electrode, −) for 100 seconds. The MUT A4 underwent thetreatment of holding the state I (the steel sheet, −; the electrode, +)for 100 seconds. As can be seen from the photographs of the MUTs A3 andA4 shown in FIG. 5 , the electrodeposition coating film 4 remainsadhered to the surface of the damaged portion 5. As can be seen from theforegoing, the treatment using only either one of the state I or IIcannot push up the electrodeposition coating film 4 on the surface ofthe damaged portion 5. This is considered because for the treatmentusing only the state I, the adherence of the electrodeposition coatingfilm 4 to the surface of the damaged portion 5 is large, and hydrogen isreleased from a broken portion of the electrodeposition coating film 4,thereby failing to contribute to the lifting of the electrodepositioncoating film 4. This is further considered because for the treatmentusing only the state II, hydrogen is not generated although dissolutionof the steel sheet 2 progresses, thereby lacking a force to push up theelectrodeposition coating film 4.

As can be seen from the results of FIG. 5 , supplying a current betweenthe electrode 12 and the steel sheet 2 while alternately switching thedirection of the current flowing through the external circuit 7 allowsadherents on the surface of the damaged portion 5 to be easily andreliably removed in a short time. This contributes to the improvement inthe reliability of the corrosion resistance test, for example.

A constant current is suitably applied between the electrode 12 and thesteel sheet 2 in order to ensure stability of the treatment in thecleaning step S4.

Alternately, a current requiring a voltage which is equal to or higherthan the theoretical voltage at which electrolysis of water occurs togenerate hydrogen is suitably applied between the electrode 12 and thesteel sheet 2. For the temperature of the water-containing material 6being 25° C., a current requiring a voltage equal to or higher than 1.23V which is a theoretical voltage at which electrolysis of water occursto generate hydrogen is suitably applied. The application of such acurrent allows electrolysis of water to be progressed in the state I,thereby ensuring a sufficient amount of hydrogen generated. Thus, thehydrogen sufficiently pushes up the adherents and improves removabilityof the adherents.

Specifically, the current value is suitably more than 1 mA and less than20 mA. The current value of 1 mA or less may require a long period oftime for removing the adherents, and may cause progress of the corrosionof the coated metal material 1. The application of the current value of20 mA or higher corresponds to the application of a voltage of 25 V orhigher, and may, particularly if the electrodeposition coating film 4has a low film quality, accelerate permeation of the water-containingmaterial 6 into a portion of the electrodeposition coating film 4 otherthan the damaged portion 5, which results in breakage of insulation ofthe electrodeposition coating film 4.

The water-containing material 6 used in the cleaning step S4 is suitablyan aqueous solution containing a supporting electrolyte. In other words,the water-containing material 6 used in the cleaning step S4 suitablydoes not contain a solid content such as the clay mineral mentionedabove. In the cleaning step S4, a material containing a larger solidcontent or a material having a high viscosity used as thewater-containing material 6 may deteriorate removability for adherentsfrom the surface of the damaged portion 5 due to the weight or viscosityof the water-containing material 6. An aqueous solution free from thesolid content and having a relatively low viscosity allows improvementin removability for the adherents.

In the cleaning step S4, the direction of the current is switchedsuitably twice or more in total. The switching from either one of thestate I or II to the other is counted as a single switching of thedirection of the current. Specifically, switching the direction of thecurrent twice in total means the same as the switching from stateI→state II→state I or the switching from the state II→state I→state II.In other words, the steel sheet 2 becomes cathode→anode→cathode oranode→cathode→anode by switching the direction of the current twice. Inthis way, at least either one of the anode reaction or the cathodereaction progresses twice or more in total on a portion of the steelsheet 2 exposed at the damaged portion 5. This improves removability foradherents from the surface of the damaged portion 5. The switchingstarts suitably from the state I. Slight progress of dissolution of thesteel sheet 2 after a small amount of hydrogen is generated allowsfurther reduction in adherence of the adherents to the surface of thedamaged portion 5.

The time for current supply in each of the states between the switchingand the subsequent switching is preferably, 1 sec or more to 60 sec orless, more preferably 3 sec or more to 45 sec or less, particularlypreferably 5 sec or more to 30 sec or less. The total time for currentsupply in the cleaning step S4 is preferably 2 sec or more to 200 sec orless, more preferably 6 sec or more to 150 sec or less, particularlypreferably 10 sec or more to 120 sec or less. The time and/or total timefor current supply in each of the states less than the lower limit maycause insufficient cleaning of the damaged portion 5, and the timeand/or total time exceeding the upper limit may cause progress of thecorrosion of the coated metal material 1 due to the current supply timethat is too long.

For the damaged portion 5 being an artificially damaged portion, theelectrodeposition coating film 4 is prone to remain on the surface ofthe damaged portion 5. However, even if the damaged portion 5 is anartificially damaged portion, adherents on the surface of the damagedportion 5 can be effectively removed in the cleaning step S4.

<<First Measurement Step>>

The size of the damaged portion 5 before the current supply step S9needs to be measured accurately. Measurement of the size of the damagedportion 5 by visual check using an image of the damaged portion 5 or thelike increases the number of processes in the test, and may increase anerror.

The first measurement step S5 is measuring the size of the damagedportion 5 using an electrochemical technique before the current supplystep S9. Specifically, as the size of the damaged portion 5, the area,diameter, or another parameter of the damaged portion 5 is measured. Thesize of the damaged portion 5 is preferably the area of the damagedportion 5 in order to easily perform the measurement.

How to measure the size of the damaged portion 5 in the firstmeasurement step S5 is specifically as follows. Specifically, as shownin the state II of FIG. 3 , a current supplier 8 supplies a constantvoltage between the electrode 12 and the steel sheet 2 serving as acathode and an anode, respectively. The current supplier 8 then detectsa current value flowing between the cathode and the anode. The currentvalue is stored as a measured value in the storage 92 of the controldevice 9. The storage 92 further stores a correlation between thecurrent value and the size of the damaged portion 5. The correlation isdetermined on an exploratory basis in advance. The arithmetic unit 91calculates the size of the damaged portion 5 based on the measuredcurrent value and the correlation.

This configuration allows the size of the damaged portion 5 to bemeasured using the electrochemical technique; this enables the processesof the test to be simplified and a measuring error to be reduced.

Description will be made with reference to specific experimental resultsshown in FIGS. 6 to 8 .

The inventors of the present application found that when the currentsupplier 8 applies a constant voltage between the electrode 12 and thesteel sheet 2 serving as a cathode and an anode, respectively in thestate II of FIG. 3 , a current value flowing between the cathode and theanode increases linearly with respect to the size of the damaged portion5.

First, a steel sheet 2 (SPC) serving as a metal base was provided withan epoxy resin-based electrodeposition coating film 4 (bakingconditions: 150° C.×20 min, thickness: 10 μm) via a chemical conversioncoating 3 (zinc phosphate coating; chemical conversion treatment time,120 sec) to produce a coated metal material 1, which was used as a MUTB. Damaged portions 5 were formed artificially in the surfaces of therespective electrodeposition coating film 4 in the MUT B using a Vickershardness tester.

Digital photomicrographs of the damaged portions 5 of the MUT B weretaken, and the areas of the damaged portions 5 were calculated as thesizes of the respective damaged portions 5 from the digitalphotomicrographs.

Further, 5 mass % salt water was adhered to the damaged portions 5 invarious sizes in the MUT B, and then, in the state II of FIG. 3 , aconstant voltage of 0.5 V was applied for 5 min, and a current value wasmeasured.

FIG. 6 is a graph showing change in the current value with time for theMUT B having a damaged portion 5 with an area of 0.62 mm². As can beseen from FIG. 6 , a fluctuation of the current value is large from thestart (0 sec) of application of the voltage to about 120 sec. This isconsidered to be because the rates of the chemical reaction at theinterfaces of the water-containing material 6 with the electrode 12 andthe steel sheet 2 is not stable for about two minutes after theapplication of the voltage, for example. After about 2 min from thestart of the application of the voltage, the current value starts tostabilize. This is considered to be because the rate of the chemicalreaction at each of the interfaces starts to stabilize. In the presentexperiment, the lowest current value during the period from 2 min to 5min after the application of the voltage was regarded as the currentvalue detected for the MUT B. For the other damaged portions 5 of theMUT B, current values were detected in the same manner Note that each ofthe current values detected is not limited to the lowest value duringpredetermined time, and may be an average or the like.

FIG. 7 is a graph obtained by plotting the current values detected inthe manner mentioned above against the area of the damaged portion 5.The data of the MUT B of FIG. 6 is indicated by a circle with a dot-dashline. As can be seen from FIG. 7 , there is a linear correlation betweenthe area of the damaged portion and the detected current value. Thecorrelation shown in FIG. 7 is an example correlation between thecurrent value and the size of the damaged portion 5, stored in thestorage 92. The correlation is determined on an exploratory basis inadvance.

FIG. 8 is a graph obtained by plotting resistance values detectedinstead of the current values against the area of the damaged portion 5.A correlation is also found between the area of the damaged portion andthe resistance value, but is a non-linear correlation. Specifically, theresistance value becomes large in a region in which the area of thedamaged portion 5 is small, whereas the resistance value becomes smallin a region in which the area of the damaged portion 5 is large.Accordingly, if the resistance value is used instead of the currentvalue, an error in fitting the regression equation and an error incalculation may become large.

The difference between FIGS. 7 and 8 is considered as follows, forexample.

The entire system in the state II of FIG. 3 includes a plurality ofinterfaces such as the interface between the electrode 12 and thewater-containing material 6 and the interface between the steel sheet 2and the water-containing material 6. Then, the resistance value of theentire system does not follow what is called the Ohm's law and does notbecome a constant. Thus, a current value, a voltage value, or aresistance value may be detected at a constant voltage or constantcurrent, in the steady state where the rate of the chemical reaction ateach of the interfaces has been stabilized after lapse of sufficienttime, and is then used to measure the size of the damaged portion 5.

For example, it is assumed that in the steady state where the rate andother factors of the chemical reaction at each of the interfaces arestable, the entire system is regarded as a single resistance, and acurrent value, a voltage value, a resistance value, and a resistivity,the length of the resistance, and the cross-sectional area of theresistance satisfy the following equations (1) and (2) of the Ohm's law.If the entire system is regarded as the resistance, R represents aresistance value, I represents a current value, V represents a voltagevalue, ρ (constant) represents a resistivity of the entire system, L(constant) represents a length of the entire system, and S represents across-sectional area of the entire system, in the equations (1) and (2).V=I×R  (1)R=ρ×L/S  (2)

Equations (1) and (2) derive equations (3) and (4).I=[V/(ρ×L)]×S  (3)V=I×ρ×L/S  (4)

As can be seen from the equation (2), the resistance value R isinversely proportional to the cross-sectional area S at each of aconstant voltage or a constant current.

As can be seen from the equation (3), the current value I isproportional to the cross-sectional area S at a constant voltage.

As can be seen from the equation (4), the voltage value V is inverselyproportional to the cross-sectional area S at a constant current.

The cross-sectional area S of the entire system is considered to have alinear correlation with the area of the damaged portion 5 if otherconditions such as the components, concentration, and other parametersof the water-containing material 6 are the same. Thus, it is predictedthat correlations of the area of the damaged portion 5 with theresistance value R, a current value I at a constant voltage, and avoltage value V at a constant current also follow the equations (2),(3), and (4). Specifically, the area of the damaged portion 5 isconsidered to have a non-linear correlation with the resistance value Rand the voltage value V at a constant current, whereas the area of thedamaged portion 5 is considered to have a linear correlation with thecurrent value I at a constant voltage. Accordingly, in order to improveaccuracy of measurement, the area of the damaged portion 5 is measuredsuitably based on the current value detected at the constant voltage.

For the diameter of the damaged portion 5 measured as the size of thedamaged portion 5, a correlation between the current value and thediameter of the damaged portion 5 such as shown in FIG. 7 may bedetermined on an exploratory basis in advance and stored in the storage92 to be used in calculation of the diameter of the damaged portion 5.

In the first measurement step S5, the constant voltage to be applied issuitably a voltage less than the theoretical voltage at whichelectrolysis of water occurs to generate hydrogen. For the temperatureof the water-containing material 6 being 25° C., a voltage less than1.23 V which is a theoretical voltage (25° C.) at which electrolysis ofwater occurs to generate hydrogen is suitably applied.

In response to the application of a constant voltage which is equal toor higher than the theoretical voltage at which electrolysis of wateroccurs to generate hydrogen, electrolysis of water progresses along withthe cathode reaction at the electrode 12. With the progress of theelectrolysis of water, an energy loss occurs due to the generation ofhydrogen. Further, the current value may be unstable due to attachmentof bubbles of hydrogen to the electrode 12 arising from the size andshape of the electrode 12, for example. The application of the constantvoltage less than the theoretical voltage at which electrolysis of wateroccurs to generate hydrogen enables a reduction in the generation ofhydrogen, thereby improving accuracy of measurement of the size of thedamaged portion 5.

The lower limit of the constant voltage may be preferably 0.05 V ormore, more preferably 0.1 V or more. The constant voltage less than thelower limit causes a too small current value, which may cause a largermeasuring error.

In the first measurement step S5, a voltage is applied suitably in thestate II (the steel sheet: +, the electrode: −) of FIG. 3 . If a voltageis applied in the state I (the steel sheet, −; the electrode, +) of FIG.3 , the cathode reaction progresses at the damaged portion 5 although acurrent flows. Thus, corrosion of the coated metal material 1 progressesbefore the current supply step S9. This may reduce the reliability ofthe corrosion resistance test. Specifically, if a voltage exceeding thetheoretical voltage at which electrolysis of water occurs is applied inthe state I (the steel sheet, −; the electrode, +) of FIG. 3 , theelectrolysis of water also progresses at the damaged portion 5 togenerate hydrogen. This causes progress of corrosion, which isunsuitable.

The area as the size of the damaged portion 5 is preferably 0.01 mm² ormore to 25 mm² or less, more preferably 0.02 mm² or more to 10 mm² orless, particularly preferably 0.05 mm² or more to 1 mm² or less. Thediameter as the size of the damaged portion 5 is preferably 0.1 mm ormore to 7 mm or less, more preferably 0.2 mm or more to 5 mm or less,particularly preferably 0.3 mm or more to 1.5 mm or less.

The size of the damaged portion 5 less than the lower limit causes a toosmall current value, which may result in insufficient correlationbetween the current value and the size of the damaged portion 5. Thesmaller the size of the damaged portion 5 is, the more the corrosion isaccelerated in the current supply step S9. However, the size of thedamaged portion 5 less than the lower limit reduces electricalconductivity in the current supply step S9, which makes it difficult forthe cathode reaction to progress. The size of the damaged portion 5exceeding the upper limit causes the cathode reaction in the currentsupply step S9 to be unstable due to the too large damaged portion 5,and causes the expansion of the electrodeposition coating film 4 to slowdown. This may reduce the reliability of the corrosion resistance test.The size of the damaged portion 5 in the range described above allowsthe size of the damaged portion 5 to be calculated accurately andeasily, and allows the corrosion resistance test to be performed in ashort time with higher reliability. Further, the progress of the cathodereaction and the progress of the expansion of the electrodepositioncoating film 4 are accelerated in the current supply step S9.

As the correlation between the current value and the size of the damagedportion 5, determined on an exploratory basis in advance, a correlationobtained using an experimental technique as shown in FIG. 7 orcalculated by an analytical technique such as a simulation may be used.

The water-containing material 6 used in the first measurement step S5may be, for example, any of the materials described as thewater-containing material 6. For example, the water-containing material6 used in the cleaning step S4 may be used as it is. For the use of adifferent water-containing material 6 in the current supply step S9, thewater-containing material 6 is replaced in the subsequent secondplacement step S6.

<<Second Placement Step>>

In the current supply step S9, the water-containing material 6 suitablyfurther contain a clay mineral and/or an additive besides water and asupporting electrolyte in order to accelerate permeation of thewater-containing material 6 into the electrodeposition coating film 4.Thus, in the second placement step S6, the water-containing material 6is replaced with the water-containing material 6 used in the currentsupply step S9.

Specifically, for example, the water-containing material 6 is aspiratedthrough the through hole 38 shown in FIGS. 1 and 2 , and newwater-containing material 6 is introduced into the water-containingmaterial holder 11.

<<Holding Step>>

Prior to the subsequent current supply step S9, the holding step S7 ofholding the water-containing material 6 for a predetermined time withthe water-containing material 6 being disposed on the surface of theelectrodeposition coating film 4 may be provided. The predeterminedtime, that is, the holding time is preferably 1 min or more to 1 day orless, more preferably, 10 min or more to 120 min or less, particularlypreferably 15 min or more to 60 min or less.

Holding the water-containing material 6 while being disposed on thesurface of the electrodeposition coating film 4 promotes, in advance,permeation of the water-containing material 6 into the electrodepositioncoating film 4. Specifically, the holding promotes, in advance, movementof ions to and permeation of water into the electrodeposition coatingfilm 4, specifically as illustrated in a dotted pattern of FIG. 3 . Thismeans that the simulated state where the corrosion resistance time hasended is reproduced in the entire measurement target portion 4A to becloser to the actual corrosion process. Accordingly, the corrosion ofthe coated metal material 1 smoothly progresses in the current supplystep S9, thereby allowing promotion of the progress of the expansion ofthe electrodeposition coating film 4 for evaluating the corrosionprogress rate representing the progress of corrosion. This enables areduction in the testing time and improvement in the reliability of thecorrosion resistance test.

In the holding step S7 and the current supply step S9, the coated metalmaterial 1 and/or water-containing material 6 are suitably warmed, orthe temperatures thereof are suitably adjusted. The temperature of thecoated metal material 1 and/or the water-containing material 6,preferably detected with the temperature sensor 37 in the subsequenttemperature measurement step S8, is preferably 30° C. or more to 100° C.or less, more preferably 50° C. or more to 100° C. or less, particularlypreferably 50° C. or more to 80° C. or less. This allows movement ofions to and permeation of water into the electrodeposition coating film4 to be accelerated. Accordingly, the corrosion resistance test can beperformed under the predetermined temperature condition with higherreliability. Specifically, for example, the control device 9 controlsthe temperatures of the rubber heater 41 and/or hot plate 43 to be inthe range described above, thereby adjusting the temperatures of thecoated metal material 1 and/or water-containing material 6.

<<Temperature Measurement Step>>

The temperature measurement step S8 is suitably performed to measure thetemperature of the water-containing material 6 prior to the currentsupply step S9. This allows the corrosion resistance test at a desiredtemperature to be performed and improves the reliability of thecorrosion resistance test.

Specifically, for example, the temperature information is detected withthe temperature sensor 37 immediately before the current supply step S9,and is then stored in the storage 92.

<<Current Supply Step>

The current supply step S9 is a step of supplying, with a currentsupplier 8, a current between the electrode 12 and the steel sheet 2serving as an anode and a cathode, respectively as shown in the state Iof FIG. 3 for corrosion of the steel sheet 2 to progress around thedamaged portion 5.

In response to the supply of a current between the electrode 12 servingas an anode and the steel sheet 2 serving as a cathode, the cathodereaction progresses in the exposing portion 5A of the steel sheet 2 atthe damaged portion 5. Then, electrolysis of water also progresses togenerate hydrogen, depending on the conditions of the current supply.

With the progress of the cathode reaction, OH⁻ is generated. This bringsthe area around the damaged portion 5 to be in an alkaline environment.This damages the under-treated surface (chemically converted surface) ofthe steel sheet 2, thereby reducing adherence of the electrodepositioncoating film 4. Accordingly, the electrodeposition coating film 4 isexpanded around the damaged portion 5. Further, hydrogen gas generatedby electrolysis of water and reduction of H⁺ accelerate the expansion ofthe electrodeposition coating film 4.

FIG. 9 shows photographs showing progress of corrosion of the coatedmetal material 1 in the current supply step S9.

First, a steel sheet 2 (SPC) serving as a metal base was provided withan epoxy resin-based electrodeposition coating film 4 (bakingconditions: 150° C.×20 min, thickness: 10 μm) via a chemical conversioncoating 3 (zinc phosphate coating; chemical conversion treatment time,120 sec) to produce a coated metal material 1, which was used as a MUTC. A damaged portion 5 was formed artificially in the surface of theelectrodeposition coating film 4 in the MUT C using a Vickers hardnesstester with a load of 30 kg.

A constant current of 1 mA was applied to the MUT C for 60 min in thestate I of FIG. 3 using 5 mass % salt water as a water-containingmaterial 6. As can be seen from FIG. 9 , hydrogen starts to be generatedat the damaged portion 5 upon start of the current supply, and theexpansion of the electrodeposition coating film 4 progresses around thedamaged portion 5 with time.

Such a progress of the cathode reaction and expansion of theelectrodeposition coating film 4 around the damaged portion 5 areaccelerated reproduction of actual corrosion of the coated metalmaterial 1. Specifically, the progress of the expansion of theelectrodeposition coating film 4 around the damaged portion 5 is asimulated progress of the corrosion of the coated metal material 1. Inthis way, the degree of progress of the corrosion of the coated metalmaterial 1 can be evaluated by evaluation of the size of the expansionof the electrodeposition coating film 4 at the time when predeterminedtime has elapsed from the start of the current supply. In particular,the rate of increase in the size of the expansion of theelectrodeposition coating film 4 corresponds to the corrosion progressrate, out of the process of corrosion of the metal described above.Accordingly, the rate of increase in the size of expansion of theelectrodeposition coating film 4 obtained as the progress degree ofcorrosion of the coated metal material 1 enables accurate evaluation ofthe corrosion resistance related to the corrosion progress rate of thecoated metal material 1.

In the current supply step S9, application of the voltage to thewater-containing material 6 bring anions (e.g., Cr) and cations (Nat) inthe water-containing material 6 to move toward the steel sheet 2 throughthe electrodeposition coating film 4. The water is then drawn withanions and cations, and permeate into the electrodeposition coating film4.

Further, the electrode 12 disposed to surround the damaged portion 5allows a voltage to be stably applied to the electrodeposition coatingfilm 4 around the damaged portion 5. This leads to efficient movement ofions to and efficient permeation of water into the electrodepositioncoating film 4 at the time of current supply.

The current supply accelerates movement of ions to and permeation ofwater into a portion of the electrodeposition coating film 4 around thedamaged portion 5 in this manner Thus, the flow of the current israpidly stabilized. Accordingly, progress of the expansion of theelectrodeposition coating film 4 at the damaged portion 5 is stabilized.

In this manner, the present embodiment allows stable acceleration of theprogress of the cathode reaction at the damaged portion 5 and of theexpansion of the electrodeposition coating film 4. This enables thecorrosion resistance test for the coated metal material 1 to beperformed accurately in a really short time.

FIG. 10 is a table showing an example of a specific corrosion resistancetest.

A steel sheet 2 (SPC) serving as a metal base was provided with an epoxyresin-based electrodeposition coating film 4 (thickness, 10 μm) via achemical conversion coating 3 (zinc phosphate coating) to produce acoated metal material 1, which was used as each of MUTs D1 to D4. Asshown in FIG. 10 , the MUTs D1 to D4 differ from each other in the filmqualities of the electrodeposition coating film 4 and the chemicalconversion coating 3 due to differences in the baking conditions for theelectrodeposition coating film 4 and the chemical conversion treatmenttime for the chemical conversion coating 3. The electrodepositioncoating film 4 having a high degree of cure was evaluated as“excellent,” and having a low degree of cure was evaluated as “poor.”The chemical conversion coating 3 was evaluated as follows: the surfaceof the steel sheet 2 after the chemical conversion treatment wasobserved with a scanning electron microscope (SEM), and if transparencywas not visually observed in a SEM image (×1500) obtained, it isevaluated as “excellent,” and if observed, it is evaluated as “poor.”

A damaged portion 5 was formed artificially in the surface of theelectrodeposition coating film 4 in each of the MUTs D1 to D4 using aVickers hardness tester with a load of 30 kg.

As experimental examples of the corrosion resistance test methodaccording to the present embodiment, a constant current of 1 mA wasapplied to the MUTs D1 to D4 for 30 min in the state I of FIG. 3 , withthe water-containing material 6 having a temperature of 65° C. Thewater-containing material 6 used was simulated mud (composition: water,1.2 L; kaolinite, 1 kg; sodium sulfate, 50 g; sodium chloride, 50 g;calcium chloride, 50 g).

Thereafter, the simulated mud was removed, the surface of each of theMUTs D1 to D4 was cleaned, and an expanded portion of theelectrodeposition coating film 4 was removed with an adhesive tape. Thepeeling diameter was then measured. FIG. 10 shows digitalphotomicrographs of the respective surfaces of the MUTs D1 to D4 afterthe peeling. Regarding the corrosion resistance of each of the MUTs, thepeeling diameter of 3 mm or less was evaluated as “excellent,” thatexceeding 3 mm and 8 mm or less was evaluated as “good,” and thatexceeding 8 mm was evaluated as “poor.”

As Reference Examples, the MUTs D1 to D4 underwent the actual corrosiontest of leaving the MUTs D1 to D4 at 50° C. and a humidity of 98% for 10days. The expansion diameter of each of the electrodeposition coatingfilms 4 was then measured. Regarding the corrosion resistance of each ofthe MUTs, the expansion diameter of 2 mm or less was evaluated as“excellent,” that exceeding 2 mm and 6 mm or less was evaluated as“good,” and that exceeding 6 mm was evaluated as “poor.”

The MUT D1 having high film qualities of both the electrodepositioncoating film 4 and the chemical conversion coating 3 had a small peelingdiameter and a small expansion diameter, and thus evaluated as“excellent” in both of the Example and Reference Example.

The MUTs D2 and D3 having a low film quality of either one of theelectrodeposition coating film 4 or the chemical conversion coating 3was evaluated as “good” in both of the Examples and Reference Examples.

The MUT D4 having a low film qualities of both the electrodepositioncoating film 4 and the chemical conversion coating 3 had a large peelingdiameter and a large expansion diameter, and thus evaluated as “poor” inboth of the Examples and Reference Examples.

As can be seen from the results shown in FIG. 10 , a sufficientcorrelation is obtained between Examples and Reference Examples, and thecorrosion resistance test method according to the present embodiment canbe used as a corrosion resistance test method with higher reliability ina short time, alternative to the actual corrosion test.

In the current supply step S9, a constant current or constant voltage,suitably a constant current is applied between the electrode 12 and thesteel sheet 2.

Under the constant current control, the current value varies a little atthe beginning of current supply, but may be controlled to beapproximately the setting value. The current supply under the constantcurrent control stabilizes the current value directly involved in theacceleration of corrosion, thereby improving the acceleratedreproducibility of corrosion. Accordingly, the reliability of thecorrosion resistance test can be improved.

In contract, under the constant voltage control, the current value mayvary greatly due to the degree of permeation of the water-containingmaterial 6 into the electrodeposition coating film 4, variations in theresistance value with deterioration or rusting of the chemicalconversion coating, and other factors, which is disadvantageous inaccelerated reproducibility of corrosion. The holding step S7 allowsacceleration of the permeation of the water-containing material 6 intothe electrodeposition coating film 4 prior to the current supply stepS9, which may reduce variations in the current value even under theconstant voltage control. The state of progress of or the degree ofcorrosion in the state of corrosion progress may be determined from theplot of current (waveform of current) under the constant voltagecontrol.

The current value in the current supply step S9 is preferably 10 μA ormore to 10 mA or less, more preferably 100 μA or more to 5 mA or less,particularly preferably 500 μA or more to 2 mA or less. The currentvalue less than 10 μA reduces accelerated reproducibility of thecorrosion, and needs a long period of time for the test. On the otherhand, the current value exceeding 10 mA makes the rate of the corrosionreaction unstable, which reduce the correlation with the progress ofactual corrosion. Setting the current value within the range describedabove achieves both a reduction in the testing time and an improvementin the reliability of the test.

The time for the current supply in the current supply step S9 may be,for example, 0.05 hr or more to 24 hr or less, preferably 0.1 hr or moreto 10 hr or less, more preferably 0.1 hr or more to 5 hr or less, inorder to shorten the testing time while obtaining sufficient spread ofthe expansion of the coating film. The time for the holding step S7 maybe preferably 0.1 hr or more to 1 hr or less.

<<Second Measurement Step>>

The size of the expansion of the electrodeposition coating film 4 afterthe current supply step S9 needs to be measured accurately. Measurementof the size by visual check using an image of the expanded portion orthe like increases the number of processes, and may increase an error.

The second measurement step S10 is measuring the size of the expansionof the electrodeposition coating film 4 around the damaged portion 5using an electrochemical technique after the current supply step S9. Asthe size of the expansion of the electrodeposition coating film 4, theexpansion diameter or expansion area, or the peeling diameter or peelingarea is measured. If the diameter of the damaged portion 5 is measuredin the first measurement step S5 as the size of the damaged portion 5,the expansion diameter or peeling diameter is measured. If the area ofthe damaged portion 5 is measured, the expansion area or peeling area ismeasured.

The size of the expansion of the electrodeposition coating film 4 ismeasured in the second measurement step S10 suitably using the samemethod as used in the first measurement step S5. This is because theaccuracy of the measured values obtained in both steps can be unified,and the calculation accuracy of the progress degree of the corrosion canbe improved.

How to measure the size of the expansion of the electrodepositioncoating film 4 in the second measurement step S10 is specifically asfollows. Specifically, as shown in the state II of FIG. 3 , a currentsupplier 8 supplies a constant voltage between the electrode 12 and thesteel sheet 2 serving as a cathode and an anode, respectively. Thecurrent supplier 8 then detects a current value flowing between thecathode and the anode. The current value is stored as a detected valuein the storage 92 of the control device 9. The storage 92 further storesa correlation between the current value and the size of the expansion ofthe electrodeposition coating film 4. The correlation is determined onan exploratory basis in advance. The arithmetic unit 91 calculates thesize of the expansion of the electrodeposition coating film 4 based onthe detected current value and the correlation.

This configuration allows the size of the expansion of theelectrodeposition coating film 4 to be measured using theelectrochemical technique; this enables the processes of the test to besimplified and a measuring error to be reduced.

Description will be made with reference to specific experimental resultsshown in FIGS. 11 to 13 .

The inventors of the present application found that when the currentsupplier 8 applies a constant voltage between the electrode 12 and thesteel sheet 2 serving as a cathode and an anode, respectively in thestate II of FIG. 3 , a current value flowing between the cathode and theanode increases linearly with respect to the size of the expansion ofthe electrodeposition coating film 4.

First, as a MUT E, a coated metal material 1 having the samespecifications as the MUT B was prepared. Damaged portions 5 were formedartificially in the surface of the electrodeposition coating film 4 inthe MUT E using a Vickers hardness tester with various loads.

Simulated mud (composition: water, 1.2 L; kaolinite, 1 kg; sodiumsulfate, 50 g; sodium chloride, 50 g; calcium chloride, 50 g) was placedas a water-containing material 6 at each of the damaged portions 5 ofthe MUT E. A constant current of 1 mA was then applied to the MUT E for30 min in the state I (steel sheet, −; electrode, +) of FIG. 3 at 65° C.of the water-containing material 6 in the current supply step S9.

With the simulated mud being placed at the damaged portion 5, a constantvoltage of 0.5 V was further applied in the state II (the steel sheet,+; the electrode, −) of FIG. 3 , and current values were then measured.

Subsequently, the simulated mud was removed, the surface of the MUT Ewas cleaned, and expanded portions of the electrodeposition coating film4 were removed. Digital photomicrographs of the peeled portion weretaken, and the peeling areas were calculated as the sizes of theexpansions of the electrodeposition coating film 4.

FIG. 11 is a graph showing change in the current value with timeobtained when a constant voltage was applied in the state II (the steelsheet, +; the electrode, −) of FIG. 3 , for the MUT E having a peelingarea calculated from the photograph of 3.1 mm². As can be seen from FIG.11 , a fluctuation of the current value is large from the start (0 sec)of application of the voltage to 120 sec. After about 2 min from thestart of the application of the voltage, the current value starts tostabilize. This is considered to be because of the stability of the rateof the chemical reaction at each of the interfaces, as in the firstmeasurement step S5. In the present experiment, the lowest current valueduring the period from 2 min to 5 min after the application of thevoltage was regarded as the current value detected at the expandedportion of the MUT E. For the other expanded portions of the MUT E,current values were detected in the same manner Note that each of thecurrent values detected is not limited to the lowest value duringpredetermined time, and may be an average or the like.

FIG. 12 is a graph obtained by plotting the current values measured inthe manner mentioned above against the peeling area. The data of the MUTE of FIG. 11 is indicated by a circle with a dot-dash line. As can beseen from FIG. 12 , there is a linear correlation between the peelingarea and the measured current value, as in FIG. 7 . The correlationshown in FIG. 12 is an example correlation between the current value andthe size of the expansion of the electrodeposition coating film 4,stored in the storage 92. The correlation is determined on anexploratory basis in advance.

FIG. 13 is a graph obtained by plotting resistance values detectedinstead of the current values against the peeling area. A correlation isalso found between the peeling area and the resistance value, but is anon-linear correlation, as in FIG. 8 . Specifically, the resistancevalue becomes large in a region in which the peeling area is small,whereas the resistance value becomes small in a region in which thepeeling area is large. Accordingly, if the resistance value is usedinstead of the current value, an error in fitting the regressionequation and an error in calculation may become large.

The difference between FIGS. 12 and 13 is considered in the same manneras for the difference between FIGS. 7 and 8 in the first measurementstep S5.

The entire system in the state II of FIG. 3 includes a plurality ofinterfaces as described above. Then, the resistance value of the entiresystem does not follow what is called the Ohm's law and does not becomea constant. Thus, a current value, a voltage value, or a resistancevalue may be detected in the steady state where the rate of the chemicalreaction at each of the interfaces has been stabilized after lapse ofsufficient time at a constant voltage or constant current, and is thenused to measure the size of the expansion of the electrodepositioncoating film 4.

In the steady state where the rate of the chemical reaction at each ofthe interfaces has been stabilized, the equations (1) to (4) are assumedto be established as described above.

Specifically, as can be seen from the equation (2), the resistance valueR is inversely proportional to the cross-sectional area S at each of aconstant voltage or a constant current.

As can be seen from the equation (3), the current value I isproportional to the cross-sectional area S at a constant voltage.

As can be seen from the equation (4), the voltage value V is inverselyproportional to the cross-sectional area S at a constant current.

When the electrodeposition coating film 4 is expanded, a gap between theelectrodeposition coating film 4 and the steel sheet 2 is formed in itsexpanded portion. This gap is considered to be filled with thewater-containing material 6 or components thereof. Then, an anodereaction progresses in a portion where the exposed surface of the steelsheet 2 in the expanded portion (i.e., corresponding to the peeledportion) is in contact with the water-containing material 6 orcomponents thereof.

The cross-sectional area S of the entire system is considered to have alinear correlation with the area of the exposed portion of the steelsheet 2 in the expanded portion, that is, the expansion area and thepeeling area if other conditions such as the components, concentration,and other parameters of the water-containing material 6 are the same.Thus, it is predicted that correlations of the expansion area or peelingarea with the resistance value R, a current value I at a constantvoltage, and a voltage value V at a constant current also follow theequations (2), (3), and (4). Specifically, the expansion area or thepeeling area is considered to have a non-linear correlation with theresistance value R and the voltage value V at a constant current,whereas have a linear correlation with the current value I at a constantvoltage. Accordingly, in order to improve the accuracy of measurement,the expansion area or peeling area of the electrodeposition coating film4 is measured suitably based on the current value detected at theconstant voltage.

For the expansion diameter or peeling diameter measured as the size ofthe expansion of the electrodeposition coating film 4, a correlationbetween the current value and expansion diameter or peeling diameter ofthe electrodeposition coating film 4 may be determined on an exploratorybasis in advance as shown in FIG. 12 and stored in the storage 92 to beused in calculation of expansion diameter or peeling diameter.

In the second measurement step S10, the constant voltage to be appliedis suitably a voltage less than the theoretical voltage at whichelectrolysis of water occurs to generate hydrogen, as in the firstmeasurement step S5. For the temperature of the water-containingmaterial 6 being 25° C., a voltage less than 1.23 V which is atheoretical voltage (25° C.) at which electrolysis of water occurs togenerate hydrogen is suitably applied. This enables a reduction in thegeneration of hydrogen, thereby improving the accuracy of measurement ofthe size of the expansion of the electrodeposition coating film 4, as inthe first measurement step S5.

The lower limit of the constant voltage may be preferably 0.05 V ormore, more preferably 0.1 V or more, as in the first measurement stepS5. The constant voltage less than the lower limit causes a too smallcurrent value, which may cause a larger measuring error.

In the second measurement step S10, a voltage is applied suitably in thestate II (the steel sheet, +; the electrode, −) of FIG. 3 , for the samereason as for the first measurement step S5. If a voltage is applied inthe state I (the steel sheet, +; the electrode, −) of FIG. 3 , thecathode reaction progresses at the damaged portion 5 although a currentflows. Thus, corrosion of the coated metal material 1 furtherprogresses. This may reduce the reliability of the corrosion resistancetest. Specifically, if a voltage exceeding the theoretical voltage atwhich electrolysis of water occurs is applied in the state I (the steelsheet, −; the electrode, +) of FIG. 3 , the electrolysis of water alsoprogresses at the damaged portion 5 to generate hydrogen. This causesprogress of corrosion, which is unsuitable.

The expansion area or peeling area as the size of the expansion of theelectrodeposition coating film 4 is preferably 0.1 mm² or more to 200mm² or less, more preferably 0.2 mm² or more to 150 mm² or less,particularly preferably 0.5 mm² or more to 120 mm² or less. Theexpansion diameter or peeling diameter as the size of the expansion ofthe electrodeposition coating film 4 is preferably 0.4 mm or more to 20mm or less, more preferably 0.6 mm or more to 17 mm or less,particularly preferably 1 mm or more to 15 mm or less.

The size of the expansion of the electrodeposition coating film 4 lessthan the lower limit causes insufficient progress of the corrosion,which may result in a reduction of the reliability of the corrosionresistance test. The size of the expansion of the electrodepositioncoating film 4 exceeding the upper limit causes too large current value,which may result in insufficient correlation with the current value. Toolarge expansion of the electrodeposition coating film 4 may requirelonger time for current supply in the current supply step S9particularly for the coated metal material 1 with a high film quality inorder to generate such a large expansion. The size of the expansion ofthe electrodeposition coating film 4 in the range described above allowsthe size of the expansion of the electrodeposition coating film 4 to becalculated accurately and easily, and allows the corrosion resistancetest to be performed in a short time with higher reliability.

As the correlation between the current value and the size of theexpansion of the electrodeposition coating film 4, determined on anexploratory basis in advance, a correlation obtained using anexperimental technique as shown in FIG. 12 or calculated using ananalytical technique such as a simulation may be used.

The water-containing material 6 used in the second measurement step S10is not limited as long as being any of the materials described above,but is suitably the water-containing material 6 used in the currentsupply step S9 as it is, in order to simplify the process of thecorrosion resistance test. In other words, it is suitable that after thecompletion of the current supply step S9, the state is changed from thestate I to the state II of FIG. 3 , and the second measurement step S10is then performed.

In a gap between the electrodeposition coating film 4 and the steelsheet 2 in the expanded portion of the electrodeposition coating film 4,hydrogen may be accumulated due to the chemical reaction during thecurrent supply. In such a case, the current value in the secondmeasurement step S10 becomes small, which may cause an increase in themeasuring error. Thus, before the second measurement step S10, a holemay be formed in the electrodeposition coating film 4 at the expandedportion to remove hydrogen.

Alternatively, regardless of the hydrogen content, the water-containingmaterial 6 may be removed after the current supply step S9, the coatedmetal material 1 may then be cleaned, the electrodeposition coating film4 in the expanded portion may thereafter be peeled off, a newwater-containing material 6 may be placed, and the second measurementstep S10 may then be performed.

<<Calculation Step>>

In the calculation step S11, the progress degree of the corrosion of thecoated metal material 1 is calculated.

As mentioned above, checking how much the electrodeposition coating film4 is expanded at the time when the predetermined time has elapsed sincethe start of current supply in the current supply step S9 allows theprogress degree of corrosion of the coated metal material 1 to beobtained.

An index representing the progress degree of corrosion includes thedifference between the size of the damaged portion 5 measured in thefirst measurement step S5 and the size of the expansion of theelectrodeposition coating film 4 measured in the second measurement stepS10, and the rate of expansion of the electrodeposition coating film 4,and is preferably the progress rate of expansion of theelectrodeposition coating film 4. This is because the progress rate ofthe expansion of the electrodeposition coating film 4 corresponds to thecorrosion progress rate.

The progress rate of expansion of the electrodeposition coating film 4is calculated as follows as the progress degree of corrosion, forexample. Specifically, based on the area or diameter of the damagedportion 5 measured in the first measurement step S5, the expansion areaor peeling area, or the expansion diameter or peeling diameter measuredin the second measurement step S10, an area of an expanded region of oran expanded distance of the electrodeposition coating film 4 during thecurrent supply is calculated. Based on the area of an expanded region orthe expanded distance, and the time for current supply in the currentsupply step S9, the progress rate of expansion of the electrodepositioncoating film 4 is calculated.

The progress degree of corrosion calculated in the calculation step S1 lcan be used to evaluate the corrosion resistance of the coated metalmaterial 1 in connection with the actual corrosion test, for example.Specifically, for example, the relationship between the progress degreeof corrosion obtained in the corrosion resistance test and the corrosionprogress rate obtained in the actual corrosion test are determined inadvance, to allow the correspondence of the result of the corrosionresistance test with the corrosion resistance of the actual corrosiontest to be checked.

Second Embodiment

Now, other embodiments according to the present disclosure will bedescribed in detail. In the description of these embodiments, the samereference characters as those in the first embodiment are used torepresent equivalent elements, and the detailed explanation thereof willbe omitted.

The first embodiment is described above with reference to the case wherethe coated metal material 1 has a single damaged portion 5, or has aplurality of damaged portions 5 at positions apart from each other, andone of the damaged portions 5 is used.

The description of the second embodiment will be made with reference tothe case where a plurality of damaged portions 5 are formed in thecoated metal material 1 at positions apart from each other, and two outof these damaged portions 5 are used.

FIG. 14 illustrates the principle of a corrosion resistance test methodaccording to the second embodiment, and an example corrosion resistancetest apparatus.

<Corrosion Resistance Test Apparatus>

The corrosion resistance test apparatus 100 shown in FIG. 14 isconfigured specifically by providing two damaged portions 5 with therespective electrode portion devices 300 shown in FIGS. 1 and 2 . Inthis case, the control device 9 may be common, or may be provided foreach of the electrode portion devices 300. If provided, a firsttemperature control element such as a rubber heater 41 is suitablyprovided for each of the electrode portion devices 300. If provided, asecond temperature control element such as a hot plate 43 may beprovided for each of the electrode portion devices 300, or at anappropriate position for the entire electrode portion devices 300.

As illustrated in FIG. 14 , the corrosion resistance test apparatus 100of the second embodiment includes two water-containing material holders11. The water-containing material holders 11 contain the respectivewater-containing materials 6. The water-containing materials 6 containedin the respective water-containing material holders 11 are in contact ofthe respective damaged portions 5.

The corrosion resistance test apparatus 100 of the second embodimentincludes two electrodes 12. The electrodes 12 are placed in therespective water-containing material holders 11, and are in contact withthe respective water-containing materials 6.

In the present embodiment, an external circuit 7 electrically connectsbetween the electrodes 12, and is not connected directly with the steelsheet 2, which differ from the corrosion resistance test apparatus 100according to the first embodiment.

<Corrosion Resistance Test Method>

<<Preparation Step>>

In a preparation step S1, a coated metal material 1 having at least twodamaged portions 5 is prepared.

At least one of the damaged portions 5 is suitably in a dot shape. Thedamaged portion 5 in the dot shape in this preparation step S1 ispreferably the damaged portion 5 with a larger size of expansion of theelectrodeposition coating film 4 measured in the second measurement stepS10 to be described later. Further, the damaged portion 5 in the dotshape is preferably the damaged portion 5 at which the cathode reactionprogresses in the current supply step S9 to be described later, i.e.,the damaged portion 5 serving as a cathode site. In this case, the shapeof the damaged portion 5 serving as an anode site is not limited toparticular shapes, and may be, for example, a linear shape such as a cutmade with a cutter.

The distance between the damaged portions 5 is preferably 2 cm or more,more preferably 3 cm or more in order to easily check the expansion ofthe electrodeposition coating film 4.

<<Circuit Connection Step and First Placement Step>>

In the circuit connection step S2, two water-containing material holders11 are disposed on the electrodeposition coating film 4 of the coatedmetal material 1 so as for the water-containing material holders 11 tosurround the respective damaged portions 5, as illustrated in FIG. 14 .Two electrodes 12 in connection with each other via wiring 71 are thenplaced in the respective water-containing material holders 11. In thefirst placement step S3, the water-containing materials 6 are introducedinto the respective water-containing material holders 11.

In this manner, two damaged portions 5 are in electrical connection witheach other using an external circuit 7 via the water-containingmaterials 6 in contact with the respective damaged portions 5.

<<Cleaning Step>>

In a cleaning step S4, the current supplier 8 supplies a current betweenthe electrodes 12 while alternately switching the direction of thecurrent flowing through the external circuit 7. Upon supply of thecurrent between the electrodes 12, the current is supplied also to thesteel sheet 2 through the water-containing materials 6.

Specifically, FIG. 14 illustrates the left electrode 12 connected to thenegative electrode side of the current supplier 8, and the rightelectrode 12 connected to the positive electrode side of the currentsupplier 8. In the state of FIG. 14 , the reduction reaction progressesat the interface of the left electrode 12 with the water-containingmaterial 6. Thus, the left electrode 12 serves as a cathode. Further,the left damaged portion 5 is in contact with the same water-containingmaterial 6 as in contact with the left electrode 12. Thus, the anodereaction progresses in the exposing portion 5A of the steel sheet 2 atthe left damaged portion 5. In other words, the left damaged portion 5serves as an anode site.

Electrons e⁻ generated by the anode reaction at the anode site move tothe right damaged portion 5 through the steel sheet 2. Then, theexposing portion 5A of the steel sheet 2 at the right damaged portion 5is in contact with the water-containing material 6, and the cathodereaction thus progresses. In other words, the right damaged portion 5serves as a cathode site. Further, the water-containing material 6 incontact with the right damaged portion 5 is also in contact with theright electrode 12. Thus, the oxidation reaction progresses at theinterface of the right electrode 12 with the water-containing material6. Accordingly, the right electrode 12 serves as an anode.

At the damaged portion 5 serving as the cathode site, the cathodereaction progresses, and the state is thus similar to the state I ofFIG. 3 . At the damaged portion 5 serving as the anode site, the anodereaction progresses, and the state is thus similar to the state II ofFIG. 3 .

Then, the direction of the current flowing through the external circuit7 is alternately switched to alternately switch between the state ofFIG. 14 and the reversed state thereof. In other words, if the directionof the current is switched twice in total from the state of FIG. 14 ,the left damaged portion 5 becomes an anode site→a cathode site→an anodesite. On the other hand, the right damaged portion 5 becomes a cathodesite→an anode site→a cathode site. Then, adherents on the two damagedportions 5 can be simultaneously removed. This improves the treatmentefficiency in the cleaning step S4.

In this case, one of the damaged portions 5 is necessarily the anodesite at first. Thus, the number of times the switching is performed issuitably three or more in total in order to ensure sufficientremovability of adherents on the damaged portions 5.

Further, in order to improve stability of the treatment, the electrodes12 and the steel sheet 2 are suitably connected via the external circuit7 to treat the damaged portions 5, in the same manner as in the firstembodiment.

<<First Measurement Step>>

With the external circuit 7 connected to the electrodes 12, one of thedamaged portions 5 serves as an anode site, and the other damagedportion 5 serves as a cathode site, as illustrated in FIG. 14 .

A first measurement step S5 may be performed in this state, but thefirst measurement step S5 is preferably performed in the same manner asin the first embodiment.

As mentioned above, at the damaged portion 5 serving as a cathode site,the corrosion may progress. In addition, a current which a smallerdamaged portion 5 out of the two damaged portions 5 can tolerate mayonly flow. Thus, if the difference in size between the damaged portions5 is large, the accuracy of measurement of the size of the largerdamaged portion 5 may decrease.

Accordingly, the first measurement step S5 of the present embodiment issuitably similar to that of the first embodiment. Specifically, theexternal circuit 7 is changed from connecting between two electrodes 12to connecting between the electrodes 12 and the steel sheet 2. Then, inthe same manner as in the first embodiment, the sizes of the two damagedportions 5 are measured.

<<Second Placement Step>>

In a second placement step S6, the external circuit 7 returns toconnecting between the electrodes 12 and an operation similar to that inthe first embodiment is performed.

<<Temperature Measurement Step>>

In a temperature measurement step S8, the temperatures of both of thewater-containing materials 6 may be measured. Particularly, thetemperature of the water-containing material 6 at the damaged portion 5serving as a cathode site (the electrode 12 on the anode side) issuitably measured.

<<Current Supply Step>

In a current supply step S9, a current is supplied between one of theelectrodes serving as an anode and the other electrode serving as acathode to bring corrosion of the coated metal material to progress.

As mentioned above, for example, in the state of FIG. 14 , the leftelectrode 12 serves as a cathode, and the right electrode 12 serves asan anode. The left damaged portion 5 serves as an anode site, and theright damaged portion 5 serves as a cathode site.

The damaged portion 5 serving as an anode site is in the state similarto the state II of the damaged portion 5 shown in FIG. 3 , as mentionedabove. At the damaged portion 5 serving as an anode site, the anodereaction progresses, and the progress of the cathode reaction isreduced. Thus, the electrodeposition coating film 4 hardly expands.

On the other hand, the damaged portion 5 serving as a cathode site is inthe state similar to the state I of the damaged portion 5 shown in FIG.3 , as mentioned above. At the damaged portion 5 serving as a cathodesite, expansion of the electrodeposition coating film 4 progresses. Inthis way, the progress degree of the corrosion of the coated metalmaterial 1 can be evaluated by evaluation of the size of the expansionof the electrodeposition coating film 4 at the cathode site at the timewhen predetermined time has elapsed.

The cathode reaction may progress also at the anode site depending onthe size, shape, and other parameters of the damaged portions 5, andconditions in current supply with the current supplier 8 such as acurrent value. Specifically, in the present embodiment, the damagedportion 5 at which the anode reaction progresses, and the damagedportion 5 at which the cathode reaction progresses out of two damagedportions 5 are suitably separated, but may not be separated clearly. Inthis case, the expansion of the electrodeposition coating film 4 mayprogress also at the anode site. In such a case, the expansion of theelectrodeposition coating film 4 may progress at both of the damagedportions 5. Thus, in the calculation step S11 to be described later, theprogress degree of corrosion of the coated metal material 1 iscalculated based on the damaged portion 5 with larger expansion of theelectrodeposition coating film 4.

In this manner, the present embodiment allows separation between theanode site at which the anode reaction progress with the current supply,and a cathode site at which the cathode reaction progresses with thecurrent supply, and further allows stable acceleration of the progressof both reactions at the respective damaged portions 5 and of theprogress of the expansion of the electrodeposition coating film 4. Thisenables a corrosion resistance test for the coated metal material 1 tobe performed accurately in a really short time.

A constant current or constant voltage, suitably a constant current isapplied between the electrodes 12 as in the first embodiment.

The current value flowing between the electrodes 12 is suitably similarto that of the first embodiment.

<<Second Measurement Step>>

A second measurement step S10 according to the present embodiment may beperformed with the external circuit 7 connected between the twoelectrodes 12, but is performed suitably in the same manner as in thesecond measurement step S10 of the first embodiment for the same reasonas for the first measurement step S5.

Specifically, the external circuit 7 is changed from connecting betweentwo electrodes 12 to connecting between the electrodes 12 and the steelsheet 2. Then, in the same manner as in the first embodiment, the sizesof the expansions of the electrodeposition coating film 4 are measured.

If the expansion of the electrodeposition coating film 4 at the cathodesite is obviously larger than that at the anode site, the size of theexpansion of the electrodeposition coating film 4 at the cathode sitemay only be measured. If the electrodeposition coating film 4 isexpanded at both of the cathode site and anode site, the sizes of theexpansion at the both may be measured and compared to select the largersize.

<<Calculation Step>>

The progress degree of corrosion of the coated metal material 1 may becalculated based on the size of the damaged portion 5 at the cathodesite and the size of the expansion of the electrodeposition coating film4. If the electrodeposition coating film 4 is expanded at both of theanode site and cathode site, the progress degree of corrosion of thecoated metal material 1 may be calculated based on the size of thedamaged portion 5 at the site with larger expansion of theelectrodeposition coating film 4 and the size of the expansion.

Third Embodiment

In the present embodiment, the following correction step S12 may beperformed.

<<Correction Step>>

A variation in the size of the damaged portion 5 before the currentsupply step S9 causes a variation in the progress degree of the cathodereaction and electrolysis of water which progress at the damaged portion5, the degree of closure of the damaged portion 5 due to expansion ofthe electrodeposition coating film 4, the degree of degassing ofhydrogen generated in the expansion of the electrodeposition coatingfilm 4, and other factors. This further causes a variation in the sizeof the expansion of the electrodeposition coating film 4, resulting in areduction of the reliability of the corrosion resistance test. However,it is difficult to prepare coated metal materials 1 having damagedportions 5 with exactly the same size in order to reduce suchvariations.

In a correction step S12, the progress degree of corrosion calculated inthe calculation step S11 is corrected based on the size of the damagedportion 5 before the current supply step S9.

Specifically, for example, the correction step S12 is performed tocorrect the progress degree of the corrosion of the coated metalmaterial 1 calculated in the calculation step S11, based on the size ofthe damaged portion 5 measured in the first measurement step S5 and acorrelation between the size of the damaged portion 5 and the progressdegree of the corrosion of the coated metal material 1. The correlationis determined on an exploratory basis in advance.

In the correction step S12, the arithmetic unit 91 of the control device9 functions as a corrector to correct the progress degree of thecorrosion of the coated metal material 1. The storage 92 further storesinformation on the corrected progress degree of the corrosion of thecoated metal material 1.

Specifically, the second embodiment will be described with reference tothe case where the progress rate of the expansion of theelectrodeposition coating film 4, i.e., the corrosion progress rate isemployed as the progress degree of corrosion. FIG. 15 is a graph showinga relationship between the diameter of each of damaged portions 5 inMUTs F1 and F2 and an index of the corrosion progress rate in thecorrosion resistance test of experimental examples to be describedlater. Note that the “index of the rate of corrosion progress” is aratio of the corrosion progress rate with respect to the rate ofcorrosion progress in the case where the diameter of the damaged portion5 is 1 mm.

As illustrated in FIG. 15 , the corrosion progress rate increases withthe decrease in the diameter of the damaged portion 5 in each of theMUTs F1 and F2 from 1.5 mm to 0.2 mm. This indicates that the smallerthe diameter of the damaged portion 5, the more the corrosion isaccelerated. In other words, the larger the diameter of the damagedportion 5, the lower the corrosion progress rate becomes, i.e., thelower the accelerated reproducibility of corrosion becomes. This isconsidered to be mainly caused by an increase in the area of theexposing portion of the steel sheet 2 at the damaged portion 5 due to anincrease in the diameter of the damaged portion 5. With the increase inthe area of the exposing portion of the steel sheet 2, anelectrochemical reaction (generation of hydrogen due to reduction ofhydrogen ions) which is not involved directly in the expansion of theelectrodeposition coating film 4 is promoted, which may increase thewaste of electrical energy supplied with the current supplier 8.

A regression equation calculated from the results in the MUTs F1 and F2is represented by a curve (R2=0.97) indicated by a solid line in FIG. 15. This regression equation is an example of the correlation mentionedabove. As described above, the correlation between the size of thedamaged portion 5 and the corrosion progress rate can be determined onan exploratory basis in advance using an experimental technique oranalytical technique such as a simulation. As the correlation,information on the regression equation indicated by a solid line in FIG.15 may be stored in the storage 92 and used for correction.

The correlation described above may be used as a correction factorcorresponding to the size of the damaged portion 5. Specifically, forexample, information on a correction factor corresponding to thepredetermined size of the damaged portion 5, calculated from theregression equation such as shown in FIG. 15 may be stored in thestorage 92 and used for correction. The correction factor is, forexample, an index of the corrosion progress rate on the regressionequation, corresponding to the predetermined diameter of the damagedportion 5 in the example of FIG. 15 . Specifically, for example, in FIG.15 , the correction factor is 1 at 1 mm of the diameter of the damagedportion 5, and is 1.5 at 0.4 mm of the diameter of the damaged portion5. Such a correction factor is calculated for the damaged portion 5 witha diameter in 0.1 mm increments and may be used for correction. Thecorrection factor corresponding to the size of the damaged portion 5calculated in advance as the correlation makes the correction easy.Accordingly, the corrosion resistance test with high reliability andversatility can be performed with a simple configuration.

As a specific example, it is assumed that the diameter of the damagedportion 5 measured in the first measurement step S5 is 0.4 mm, thecorrosion progress rate calculated in the calculation step S11 is 1.5mm/h. Further, the correction factor is used as the correlation, and forexample, the correction factor is 1 at 1 mm of the diameter of thedamaged portion 5, and is 1.5 at 0.4 mm of the diameter of the damagedportion 5. In this case, the arithmetic unit 91 corrects the corrosionprogress rate of 1.5 mm/h to 1 mm/h by dividing 1.5 mm/h by 1.5 which isa correction factor, based on information on the diameter of the damagedportion 5 being 0.4 mm and information on the correction factor being1.5 at 0.4 mm of the diameter of the damaged portion 5 read out from thestorage 92.

The correction step S12 allows accurate evaluation of the progressdegree of corrosion of the coated metal material 1 regardless of thesize of the damaged portion 5 where the cathode reaction progresses,measured before the current supply. Accordingly, the reliability andversatility of the corrosion resistance test can be enhanced.

Experimental Examples

[Corrosion Resistance Test]

As shown in Table 2, two kinds of MUTs which differ from each other inpaint of the electrodeposition coating film 4 and the electrodepositionbaking condition were prepared as MUTs F1 and F2.

TABLE 2 Material Under Test F1 F2 Electrodeposition 160° C. × 10 min140° C. × 20 min Baking Conditions Diameter of Damaged 0.2 0.2 Portion(mm) — 0.42 0.6 0.6 — 1 1.5 1.5 Temperature (° C.) 65 65 Holding Time(min) 30 30 Time for Current 0.5 0.5 Supply (hr)

Each of the MUTs F1 and F2 uses a steel sheet 2 as a metal base, and azinc phosphate coating (chemical conversion treatment time, 120 sec) asa chemical conversion coating, and an electrodeposition coating film 4with a thickness of 10 μm. The MUTs F1 and F2 underwent the corrosionresistance test according to the present embodiment in the configurationshown in FIG. 14 .

In each of the MUTs F1 and F2, two damaged portions 5 with the samediameter reaching the steel sheet 2 were formed at a distance of 4 cmfrom each other using a Vickers hardness tester. Specifically, as shownin Table 2, for MUT F1, three kinds of samples each having two damagedportions 5 with a diameter of 0.2 mm, 0.6 mm, or 1.5 mm were prepared.For MUT F2, five kinds of samples each having two damaged portions 5with a diameter of 0.2 mm, 0.42 mm, 0.6 mm, 1 mm, or 1.5 mm wereprepared.

The water-containing material 6 used was simulated mud obtained bymixing 50 g of sodium chloride as a supporting electrolyte, 50 g ofcalcium chloride, 50 g of sodium sulfate, and 1000 g of kaolinite as aclay mineral with respect to 1.2 L of water. The electrode 12 used was aring-shaped perforated electrode (made from platinum) with an outerdiameter of about 12 mm and an inner diameter of about 10 mm A hot platewas disposed below the steel sheet 2, and the steel sheet 2 and thewater-containing material 6 were warmed to 65° C.

A current value supplied with the current supplier 8 was 1 mA. Thewater-containing material 6 being placed on the surface of theelectrodeposition coating film 4 was held for 30 min, and then a currentwas supplied. The time for the current supply was 0.5 hr.

After the end of the current supply, the corrosion progress rateillustrated in FIG. 15 was calculated for each of the MUTs by the methodmentioned above.

Fourth Embodiment

In the embodiments described above, the first measurement step S5 ismeasuring the size of the damaged portion 5 using an electrochemicaltechnique, but is not limited thereto.

Specifically, for example, the size of the size of the damaged portion 5may be measured based on image data on the surface of the coated metalmaterial 1.

Specifically, the first measurement device may include an image detectorfor acquiring image data on the surface of the coated metal material 1,i.e., the electrodeposition coating film 4. Specific examples of theimage detector include a camera such as a CCD camera, a digitalmicroscope, an optical microscope, and an electron microscope. The imagedetector takes an image of the damaged portion 5 before the currentsupply step S9 in the first measurement step S5.

In this case, the image detector may be electrically or wirelesslyconnected to the control device 9. The image data acquired with theimage detector is transmitted to the control device 9 and stored in thestorage 92. The arithmetic unit 91 measures the size of the damagedportion 5 on the image data. This configuration uses image data acquiredwith the image detector, and thus enables accurate measurement of thesize of the damaged portion 5. Note that the control device 9 may alsobe configured to output a control signal to an image detector to controlthe timing of taking an image with a camera.

Fifth Embodiment

In the embodiments described above, the measurement target portion 4A ofthe coated metal material 1 has a damaged portion 5 reaching the steelsheet 2 through the electrodeposition coating film 4 and the chemicalconversion coating 3, but the damaged portion 5 may not reach the steelsheet 2. The measurement target portion 4A may not have a damagedportion 5.

In this case, for example, when the water-containing material 6 ispermeated through the electrodeposition coating film 4 and then reachesthe steel sheet 2, a cathode reaction (state I) or an anode reaction(state II) progresses in a contact portion between the water-containingmaterial 6 and the steel sheet 2 in the measurement target portion 4Ashown in FIG. 3 . Specifically, if the damaged portion 5 does not reachthe steel sheet 2, or no damaged portion 5 is formed, the end of thecorrosion resistance time is considered to be when the water-containingmaterial 6 first reaches the steel sheet 2 after permeating through theelectrodeposition coating film 4. Upon contact of the water-containingmaterial 6 with the steel sheet 2, corrosion starts to occur. Then, thecorrosion progresses from a portion where the corrosion first occurs,and the corrosion progress rate is calculated based on the size of theexpansion of the electrodeposition coating film 4. The coated metalmaterial 1 suitably has a damaged portion 5, particularly reaching thesteel sheet 2, in order to accelerate corrosion from the desiredposition.

For the coated metal material 1 having no damaged portion 5, thecleaning step S4 and first measurement step S5 are unnecessary. In thiscase, for example, the size of the expansion of the electrodepositioncoating film 4 occurred in the current supply step S9 may be measured inthe second measurement step S10, and the progress degree of corrosionmay be calculated based on the size.

Other Embodiments

The electrode portion device 300 and the corrosion resistance testapparatus 100 shown in FIGS. 1 and 2 are mere examples, and theconfigurations of the electrode portion device and the corrosionresistance test apparatus are not limited to those of FIGS. 1 and 2 aslong as the principle of the corrosion resistance test method shown inFIGS. 3 and 14 and described in the embodiments are implementable.

The embodiments described above each include a control device 9connected electrically or wirelessly to various detectors and targets tobe controlled, but the corrosion resistance test method according to thepresent disclosure may be performed with other units. Specifically, forexample, current supply information of the current supplier 8,temperature information of the temperature sensor 37, image data of theimage detector, and other information may be read by the user intoanother computer to perform the process.

In the embodiments described above, for example, a single control device9 functions as a calculator in the first measurement step S5, secondmeasurement step S10, and calculation step S11, but, for example,different control devices may be used as units for the respective steps.A single control device 9 suitably functions to perform multiple rolesin order to improve accuracy of the results of calculation with thecontrol device 9 and contribute to the downsizing of the corrosionresistance test apparatus 100.

The adjustment of the temperatures of the coated metal material 1 andthe water-containing material 6 is not limited to the configurations ofthe embodiments described above. For example, the electrode portiondevice 300 may be introduced into a furnace to adjust the temperatures.

The second placement step S6 may be performed prior to the firstmeasurement step S5. For the current supply step S9 and secondmeasurement step S10 using different water-containing materials 6, athird placement step similar to the second placement step S6 may beperformed between the current supply step S9 and the second measurementstep S10. Only either one of the second placement step S6 or the thirdplacement step may be performed. For the cleaning step S4, firstmeasurement step S5, current supply step S9, and second measurement stepS10 using the common water-containing material 6, none of the secondplacement step S6 and third placement step may be performed.

The present disclosure enables a highly reliable and simple measurementmethod and measurement device for measuring a size of expansion of asurface treatment film with high versatility, and a corrosion resistancetest method and corrosion resistance test apparatus for a coated metalmaterial, to be provided, and is thus quite useful.

What is claimed is:
 1. A measurement method for measuring a size ofexpansion of at least one damaged portion of a surface treatment filmoccurred in a coated metal material that includes a metal base and thesurface treatment film provided on the metal base, the measurementmethod comprising the steps of: disposing a water-containing material tobe in contact with the at least one damaged portion and an electrode tobe in contact with the water-containing material, and electricallyconnecting between the electrode and the metal base with an externalcircuit; applying, with the external circuit, a constant DC voltagebetween the electrode and the metal base, as a cathode and an anode,respectively, and measuring a current value flowing therebetween; andcalculating, with a calculator, a size of the expansion of the at leastone damaged portion based on a lowest value or an average value duringpredetermined time of the current value measured and a correlationbetween the current value and the size of the expansion of the at leastone damaged portion, the correlation being determined on an exploratorybasis in advance.
 2. The measurement method of claim 1, wherein theconstant DC voltage is less than a theoretical voltage at whichelectrolysis of water occurs to generate hydrogen.
 3. The measurementmethod of claim 2, wherein the constant DC voltage is less than 1.23 V.4. The measurement method of claim 3, wherein the size of the expansionof the at least one damaged portion is an area of the expansion of theat least one damaged portion, and the area of the expansion of the atleast one damaged portion is 0.1 mm² or more to 200 mm² or less.
 5. Themeasurement method of claim 4, wherein the surface treatment film is aresin coating film.
 6. A measurement device for measuring a size ofexpansion of the at least one damaged portion of a surface treatmentfilm occurred in a coated metal material that includes a metal base andthe surface treatment film provided on the metal base, the measurementdevice comprising: an electrode to be in contact with a water-containingmaterial disposed to be in contact with the at least one damagedportion; an external circuit configured to electrically connect betweenthe electrode and the metal base; a current supplier provided on theexternal circuit and configured to supply a constant DC voltage betweenthe electrode and the metal base as a cathode and an anode,respectively; a current detector configured to detect a current valueflowing between the electrode and the metal base; and a calculatorconfigured to calculate a size of the expansion of the at least onedamaged portion, based on the lowest value or an average value duringpredetermined time of the current value detected and a correlationbetween the current value and the size of the expansion of the at leastone damaged portion, the correlation being determined on an exploratorybasis in advance.
 7. The measurement device of claim 6, wherein theconstant DC voltage is less than a theoretical voltage at whichelectrolysis of water occurs to generate hydrogen.
 8. The measurementdevice of claim 7, wherein the constant DC voltage is less than 1.23 V.9. A corrosion resistance test method for a coated metal material thatincludes a metal base and a surface treatment film provided on the metalbase, the corrosion resistance test method comprising the steps of:preparing a coated metal material having the at least one damagedportion which is one or more damaged portions reaching the metal basethrough the surface treatment film; disposing a water-containingmaterial to be in contact with one or two out of the one or more damagedportions and one or two electrodes to be in contact with thewater-containing material, and electrically connecting between theelectrode and the metal base, or between the two electrodes, with anexternal circuit; measuring a size of one or two out of the one or moredamaged portions; supplying, with the external circuit, a currentbetween the electrode and the metal base, or between one of the twoelectrodes and the other, as an anode and a cathode, respectively toexpand the surface treatment film around the one or two out of the oneor more damaged portions; measuring a size of expansion of the one ortwo out of the one or more damaged portions of the surface treatmentfilm using the measurement method of claim 1; and calculating a progressdegree of corrosion of the coated metal material, based on the size ofthe one or two out of the one or more damaged portions and the size ofthe expansion of the one or two out of the one or more damaged portions,wherein the progress degree of corrosion is a difference between thesize of the one or two out of the one or more damaged portions and thesize of the expansion of the one or two out of the one or more damagedportions of the surface treatment film, or a progress rate of peeling ofthe surface treatment film.
 10. The corrosion resistance test method ofclaim 9, further comprising the step of correcting the calculatedprogress degree of the corrosion of the coated metal material, based onthe size of the one or two out of the one or more damaged portions and acorrelation between the size of the damaged portion and the progressdegree of the corrosion of the coated metal material, the correlationbeing determined on an exploratory basis in advance.
 11. The corrosionresistance test method of claim 10, wherein the progress degree ofcorrosion is a rate of increase in the size of the expansion of the oneor two out of the one or more damaged portions of the surface treatmentfilm.
 12. The corrosion resistance test method of claim 9, wherein theone or more damaged portions are one or more artificially damagedportions.
 13. The corrosion resistance test method of claim 12, whereinthe one or more artificially damaged portions are in a dot shape in aplan view.
 14. A corrosion resistance test apparatus for a coated metalmaterial that includes a metal base and a surface treatment filmprovided on the metal base, the corrosion resistance test apparatuscomprising: one or two electrodes to be in contact with awater-containing material disposed to be in contact with the at leastone damaged portion which is one or two out of one or more damagedportions reaching the metal base through the surface treatment film; anexternal circuit configured to electrically connect between theelectrode and the metal base, or between the two electrodes; anadditional measurement device for measuring a size of the one or two outof the one or more damaged portions; a current supplier provided on theexternal circuit and configured to supply a current between theelectrode and the metal base, or between one of the two electrodes andthe other, as an anode and a cathode, respectively to expand the surfacetreatment film around the one or two out of the one or more damagedportions; the measurement device of claim 6, for measuring the size ofthe expansion of the one or two out of the one or more damaged portionsof the surface treatment film; and a calculator configured to calculatea progress degree of corrosion of the coated metal material, based onthe size of the one or two out of the one or more damaged portions andthe size of the expansion of the one or two out of the one or moredamaged portions, wherein the progress degree of corrosion is adifference between the size of the one or two out of the one or moredamaged portions and the size of the expansion of the one or two out ofthe one or more damaged portions of the surface treatment film, or aprogress rate of peeling of the surface treatment film.
 15. Thecorrosion resistance test apparatus of claim 14, further comprising: acorrector configured to correct the progress degree of the corrosion ofthe coated metal material calculated by the calculator, based on thesize of the one or two out of the one or more damaged portions and acorrelation between the size of the damaged portion and the progressdegree of the corrosion of the coated metal material, the correlationbeing determined on an exploratory basis in advance.
 16. The measurementmethod of claim 1, wherein the constant DC voltage is less than 1.23 V.17. The measurement method of claim 1, wherein the size of the expansionof the at least one damaged portion is an area of the expansion of theat least one damaged portion, and the area of the expansion of the atleast one damaged portion is 0.1 mm² or more to 200 mm² or less.
 18. Themeasurement method of claim 1, wherein the surface treatment film is aresin coating film.
 19. The measurement device of claim 6, wherein theconstant DC voltage is less than 1.23 V.
 20. The corrosion resistancetest method of claim 9, wherein the progress degree of corrosion is arate of increase in the size of the expansion of the one or two out ofthe one or more damaged portions of the surface treatment film.